My wife is my almost-exclusive dive buddy. We have similar gear, but not identical. It hasn't been a problem.
At this point I'd say that we probably know each other's gear as well as our own (including swapping gear in the pool at times just for something to do). We certainly know it better than if we end up buddied with someone different for some reason.
- BC's: She has a DUI back-inflate. I have a BP/W.
- Regs: Both have Atomic B2 primaries, but she has an SS1 octo; I have a Z2 octo. Benefit here is that they're on the same maintenance schedule, and can be serviced at the same place. She likes the one-less-hose config of the SS1; I prefer a traditional octo and inflator.
- Computers: I picked up a pair of Atmos 2's when Scubatoys had them on sale back when we started diving. Main benefit is that we can share a computer interface, and use the same logging software. She saw the Liquivision Keon at BTS last month though and fell in love with it, so i suspect she'll be changing that out at some point. She never quite got the hang of the 2-button Aeris interface.
- Exposure protection - we both started diving wet, and we've kept that pretty much aligned so we're compatible with water temps. I just bought a drysuit, which she's not ready for, so things may get interesting there
- Fins - personal preference, but we did both upgrade to spring straps.
At this point I'd say that we probably know each other's gear as well as our own (including swapping gear in the pool at times just for something to do). We certainly know it better than if we end up buddied with someone different for some reason.