Just played it, and first I should say I'm glad to at least see people trying to make coop maps. This is a start for you I know, so I hope you take what I have to say constructively and go on to make more maps.
First, If when you say "You took the viewing room instance from PeTI", you mean you built everything else in hammer, then you have done a great job keeping your textures lined up properly. Although I have to say this really feels like a full on PeTI map converted to coop in hammer, rather then a simple use of a single instance and the map built from scratch. It is somewhat hard to tell what you implied in the description.
Second, the entire puzzle is huge. The puzzle it self could have been done in almost 1/4 the space (not including the [spoiler]faith plate collision parts, which should have been the only open area parts[/spoiler]). It was quite frustrating walking across the entire main room to check out each area, and then to get to them in order once we have figured out whats going on. And even then, there were some unnecessary long hallways and such. I almost ended up feeling like I spent more time walking back and forth then I did solving puzzle.
Next, is you have VERY minimal to no lighting at all in certain areas, if it weren't for the default cubemaps being so bright it would have been almost impossible to see. If you had proper cubemaps in the hallways, you probably wouldn't be able to see anyways. If you don't end up making the map smaller, I would consider adding a lot more glass strip lighting. Maybe consider filling whole panels with 4 strips.
Also, you basically made the only white panels in the whole map exactly what you needed to do to solve the puzzle. You can make a map more attractive by adding white areas that don't hurt the intended solution of the map. This will also help brighten your map up.
Now, to on to specific puzzle elements so I'm really going to start spoiling. First, the arrangement of the "rooms" are ordered odd and made figuring things out a bit frustrating. [spoiler]The first room to your right being one of the last things to be done was one of the first things I spent time doing, to no success.[/spoiler]. This could also just be frustrating because it took forever to walk to the next room and back. There are overlay signs with dots that can indicate which area should be explored first if ordering the rooms differently is not an easy option. (filter: sign or dot)
There is also a problem with faith plates that takes the coop out of this map a bit. [spoiler]The faithplate collision to get the cube to its button can be done by one person easy. Just turn a little in the air and hit the wall. You will land right on the platform by your self.[/spoiler] However, you have to do this in two areas of the map, and this bug doesn't occur in the first area. Only towards the exit.
And seriously... what is with the [spoiler]pit of acid right before the exit door[/spoiler]. It's like the most unnecessary thing ever. Being as small as it is doesn't prevent anyone from [spoiler]jumping to the door right over it from the side[/spoiler], and seeing as once you have the door open an [spoiler]elevator comes up to fill the pit in for your safety[/spoiler]. Maybe if it was larger and had a separate puzzle element attached to it could it be useful, but for right now it seemed very out of place. Almost troll-ish.
That cube dropper.... Way up there.... is it a button? is it a... I don't know, I couldn't see it, it was so far away and had nothing lighting it. The puzzle element triggering the cube dropper doesn't even really have an obvious indicator for it either. It wasn't until I finally walked by and saw a cube did I realize what it was after I finally triggered it.
Now, if your still reading this, I hope you continue to make maps (hopefully more coop 
). The puzzle concepts, although somewhat simple, are a good start and I enjoyed my short playthrough. Although, the next time you let me trap my partner in a room with my choice of letting him out though, he might kill you... or me.
[EDIT]
Holy... Long post is long. 
Just played it, and first I should say I'm glad to at least see people trying to make coop maps. This is a start for you I know, so I hope you take what I have to say constructively and go on to make more maps.
First, If when you say "You took the viewing room instance from PeTI", you mean you built everything else in hammer, then you have done a great job keeping your textures lined up properly. Although I have to say this really feels like a full on PeTI map converted to coop in hammer, rather then a simple use of a single instance and the map built from scratch. It is somewhat hard to tell what you implied in the description.
Second, the entire puzzle is huge. The puzzle it self could have been done in almost 1/4 the space (not including the
Spoiler
faith plate collision parts, which should have been the only open area parts
). It was quite frustrating walking across the entire main room to check out each area, and then to get to them in order once we have figured out whats going on. And even then, there were some unnecessary long hallways and such. I almost ended up feeling like I spent more time walking back and forth then I did solving puzzle.
Next, is you have VERY minimal to no lighting at all in certain areas, if it weren't for the default cubemaps being so bright it would have been almost impossible to see. If you had proper cubemaps in the hallways, you probably wouldn't be able to see anyways. If you don't end up making the map smaller, I would consider adding a lot more glass strip lighting. Maybe consider filling whole panels with 4 strips.
Also, you basically made the only white panels in the whole map exactly what you needed to do to solve the puzzle. You can make a map more attractive by adding white areas that don't hurt the intended solution of the map. This will also help brighten your map up.
Now, to on to specific puzzle elements so I'm really going to start spoiling. First, the arrangement of the "rooms" are ordered odd and made figuring things out a bit frustrating.
Spoiler
The first room to your right being one of the last things to be done was one of the first things I spent time doing, to no success.
. This could also just be frustrating because it took forever to walk to the next room and back. There are overlay signs with dots that can indicate which area should be explored first if ordering the rooms differently is not an easy option. (filter: sign or dot)
There is also a problem with faith plates that takes the coop out of this map a bit.
Spoiler
The faithplate collision to get the cube to its button can be done by one person easy. Just turn a little in the air and hit the wall. You will land right on the platform by your self.
However, you have to do this in two areas of the map, and this bug doesn't occur in the first area. Only towards the exit.
And seriously... what is with the
Spoiler
pit of acid right before the exit door
. It's like the most unnecessary thing ever. Being as small as it is doesn't prevent anyone from
Spoiler
jumping to the door right over it from the side
, and seeing as once you have the door open an
Spoiler
elevator comes up to fill the pit in for your safety
. Maybe if it was larger and had a separate puzzle element attached to it could it be useful, but for right now it seemed very out of place. Almost troll-ish.
That cube dropper.... Way up there.... is it a button? is it a... I don't know, I couldn't see it, it was so far away and had nothing lighting it. The puzzle element triggering the cube dropper doesn't even really have an obvious indicator for it either. It wasn't until I finally walked by and saw a cube did I realize what it was after I finally triggered it.
Now, if your still reading this, I hope you continue to make maps (hopefully more coop ). The puzzle concepts, although somewhat simple, are a good start and I enjoyed my short playthrough. Although, the next time you let me trap my partner in a room with my choice of letting him out though, he might kill you... or me.
[EDIT]
Holy... Long post is long.
