Is it any better to put down two layers of 3/4" plywood or is it better to put down one sheet of 1 1/2". I have to have it that thick to match the floor height of the other room?

 

Where can you find 1 1/2"? I don't know that I have ever seen that

 

I had to do that to my living rm [1/2 of the rm was an addition built prior to my purchase] To save a few bucks, I stripped the floor with 3/4" and then covered it with 1/2" [which brought it level] and then installed 3/8" over both new and old. That's been 15+ yrs ago and no problems yet

I've seen 1" plywood but it was very expensive, never seen any ply thicker than that.

 

If you need 1 1/2" then use 2 layers of 3/4" and stager the joints.
Glue and Screw.
Is this in 1 room or are you meeting another...(height wise)?

 

Ok, sounds good.
Yes, it is where one room meets another.

 

..So, I gather, the 1-1/2"s (or 2 - 3/4" ply) will meet the height of the other room's floor.
Are you planning on putting in a transition cap to bridge the 2 rooms.? -or run flooring thru the 2 rooms?

 

We have not decided for sure what type of floor will go on it yet.

 

Then that should be something to consider right there.
You have a chance to select a subfloor height that will result in a flush finished floor. I'd say wait till you decide on what you'll put down.

The 3/4 plywood will already take care of the structural integrity. So you can put that down even before you decide.

 

Is there an advantage to gluing and screwing instead of just screwing?

Also, what is the proper distance between screws?

 

Yes. Gluing adds additional strength and rigidity to the floor.
Normally, min 12" apart.