Your question may be brand specific, my Cochran GEMINI A.I. hoseless computers absolutely WILL detect/communicate with each other (assuming they're within range of each other) regardless of whether or not they're hooked up to a tank or pressurized, the wrist unit will read 0 psi and be 'flashing' once I 'wake up' the tank unit and the wrist unit. I'd HIGHLY suggest you do a proper check out of the unit to confirm there are no 'issues' and the unit is operating as designed, this includes someway/somehow getting your hands on a full scuba tank (at the dive shop if needed) and testing your reg set for basic 'soundness'. I just spent about 3 hrs in the pool at a LDS this afternoon testing out 2 complete reg sets (each with 2 computers) to confirm there are no obvious 'issues'. Always a good idea to replace computer batteries before the 'big' trip, sucks to have your $ 2500 computer felled by a flat battery and you're diving in the middle of nowhere with spares unavailable (especially if your unit uses a wierd/proprietary/odd size battery)
I'm heading to Belize for a week 7/16-23 (Sun Dancer) and don't want any of my cool 'toyz' out of action!