This is an old problem, where the piano may well be in tune with itself, but not up to concert pitch - assuming it's an acoustic piano, in which case, it may not even be in tune with itself! There is no point in using guitar tuners for the guitars, so the reference point has to be the piano. Play the appropriate notes on the piano, and tune the guitars to those.
Before the days of electronic pianos, my band would use the oft available, oft slightly out of tune piano at the venue, and we all had to re-tune to that. Use the tuners to ascertain what the piano is tuned to as a matter of interest.