Can I use the same cold air return duct in a wall between two rooms? For example, the duct would run down the wall between the studs and then could I put the register at the bottom of the wall on both sides giving both rooms access to the same return duct?
I see no problem in doing that, as long as the return duct is sized big enough for the both rooms. I would think it would need to be at least the size of both room supply ducts combined.
An alternative would be to use a Jumper duct.
Here is the solution that I am going to use:
Divide the space in two vertically with another 2x4 stud. Pan opposite sides of the studs so that the rooms each have their own duct. The I will put the grille on both walls at the bottom but only half of the grille will actually be pulling air. The other half will be blocked by the sheetrock.
The picture below illustrates from the top view: