Is it sufficient to disconnect the negative terminal of a voltage source?
My original thought was that your N channel MOSFETs are upside down because of the way you have the on/off switch arrangement. However, after @ThePhoton pointed out my blindness, another problem arose in the "turning off" area. To turn-off the MOSFET you do need to have a different arrangement for the "switch" you have drawn. The disconnected pin on SW3 (or SW4) has to be connected to the source terminal on the MOSFET or it won't properly shut down "off" - they'll conduct with 1 or 2 volts across them.
Generally most MOSFETs are OK with a 12 volt gate-source signal but some may not be.
Presumably your batteries are safe to be used in parallel.


Other than that, I'd have no problem doing it this way providing there isn't an electrical path from the negative battery terminal going somewhere else that you have not shown in your picture. The extra connection due to the wrongly connected switch will of course be another connection so some care is needed here to make sure it works correctly and doesn't leak current and discharge the battery.


  Is it sufficient to disconnect the negative terminal of a voltage
  source?


It's sufficient in many applications.