After further testing with a character using Master Critical Strike with a Keen Short Lightsaber (Nextor crystal), I'm now pretty sure Master Criticals: 2d6 (+2-12) is added to any lightsaber which already has a Massive Criticals bonus:
Massive Criticals: 1d3 (+1-3) added bonus damage 3-15
Massive Criticals: 1d4 (+1-4) added bonus damage 3-16
Massive Criticals: 1d6 (+1-6) added bonus damage 3-18
Massive Criticals: 1d8 (+1-8) added bonus damage 3-20
Massive Criticals: 1d10 (+1-10) added bonus damage 3-22
Massive Criticals: 1d12 (+1-12) added bonus damage 3-24
Combining a black Name Crystal focused at level < 12 (Massive Criticals: 1d4 (+1-4)) and a Discharge Energy Cell (Massive Criticals: 1d3 (+1-3)) added bonus damage 5-18, within the now expected range of 4-19 (the chance of 4 or 19 is 1/432 apiece), so I'm also pretty sure that this bonus is only added once, and not once for every Massive Criticals bonus.
As far as I can tell, the only prerequisites are using a lightsaber with a Massive Criticals bonus (any off hand (Short) Lightsaber must have its own bonus for this to be added to it as well): unlike Neverwinter Nights' overwhelming critical, level 21, Strength > 22, a Keen bonus (Improved Critical) and Power Attack aren't required.