Where were those located?
garage
There has to be at least one stud between them, otherwise the studs supporting the drywall would exceed the maximum 24" on center. Not only that, but if there is no stud between them, how are they supported - hanging off one stud to one side?
good question and i did not think of that.

If there is a stud, there should be drywall on it (Example: If in the garage, then that wall needs to have 1/2" gypsum board "separation" to the living space.) 1/2 in. interior and exterior? there is drywall on the back side, just missing at the garage side.

Was the gap between them only back to the stud? Into the wall cavity?
into the cavity

Looking at the photo, it looks there is a void between them, with a PVC raceway connecting them together near the bottom. The left one looks like it is "service equipment" and the right one looks like it is "not service equipment". You are correct