We use Macdive since 2009 now and it is a great app. Now on iOS devices is wonderfull. Nevertheless I think signature is one of the 2 only thing that miss in Macdive. It is important to have signature on your dives here in France if you want to pass a new grade and it is possible only on Divelog rig...
My wife andme or both divers. We use the same kind of computer (Suunto Gekko) and back up data on our (very) old "Power book G4 Titanium 500 MHz" ( yes it still working, and under OS X.5!...).
How to have 2 accounts? Actually we sort The file by adding a specific name in a field...