momentary to flip flop momentary

It can be done using a D flip flop in toggle mode.  Search for a D flip flop toggle circuit and try to find one with a debounced input - you'll most likely need it. 

On the outputs of the flip flop you'll need some timing circuitry that outputs a single pulse each time the button is pushed for a predetermined amount of time.  That should be fairly easy to find schematics for, as well.  I believe the12volt even has a relay version of a constant to single pulse relay circuit that could be a basis for what you are doing.  You wouldn't want to include the relays though as that would drain the battery.

Do you want to sync the single pushbutton up so that it always performs the correct function (ie if the doors are already locked it will unlock them intead of trying to relock them if lock is the opposite state that the circuit it is)?  If so, that will be a bit more difficult.