I have seen answers to similar questions but not sure they address my situation. I recently "installed" 2 separate 20A circuit runs for a dishwasher and a garbage disposal respectively (overkill perhaps). I will be installing a switch between the breaker and the garbage disposal receptacle next. I will then install the receptacle underneath the kitchen sink where the garbage disposal and dishwasher will plug in. I believe I have read that NEC 2014 requires GFCI for a dishwasher but not for disposal (seems backward to me but okay). If so, does this mean I should install a 2-gang box with one GFCI duplex receptacle for the dishwasher and then one standard (or another GFCI) duplex receptacle for the disposal? Can I use one GFCI duplex receptacle for 2 separate circuits?
Yes, you are right. Current code wants the dishwasher GFCI protected. However, unlike the dishwasher, there is no specific article that explicitly states a disposal requires one.
A Double pole GFCI 20 Amp breaker can protect two individual appliances. The Wiring is known as a MWBC "multi-wire branch circuit" and uses 12-3 "Black, Red, White, and Ground".
However, current code also wants everything in a kitchen, including dishwashers to be ARC-fault protected. One solution is to use a Double-Pole Dual GFCI/ARC fault 20 Amp breaker.
But since you ran two separate branch circuits I don't think code allows you to combine the neutrals as one in the panel because the conductors are not in the same sheath/raceway when they leave the panel. In which case, you would just do as you mentioned using a 2-gang box with one GFCI and one regular outlet, which is probably far way less expensive!
You can't use one GFCI receptacle for two circuits. You could use two GFCI receptacles in a 2-gang box but since it will probably be way back in the back of the cabinet, that probably wouldn't be considered "readily accessible." It's probably most practical to use GFCI breakers to meet this requirement. Another possibility is to install deadfront GFCIs in an accessible area ahead of the disposal and dishwasher receptacles.