Yes, that's called cooking PiP (pot-in-pot, pan-in-pot) and may be done for various situations (or anytime).

Since there are only 2 of you any of the 6 qt models would be plenty though (unless you like to entertain or "prep" large amounts then freeze, etc, etc). Personally I like the smaller size because the footprint and height will be less and the pot will be lighter-weight and smaller when cleaning or carrying (especially if not doing PiP).

The best-selling model of all time is the DUO60 and it's just a few dollars more (if it doesn't go on sale the rest of today like we all thought it would, again!):
https://www.amazon.com/Instant-Pot-Multi-Use-Programmable-Pressure/dp/B00FLYWNYQ

And the LUX line also has a 6 qt (as well as perhaps a few 5 qt units here and there) for less money but those are 6-in-1's rather than 7-in-1's so won't have the Yogurt mode (which can also be used to proof dough and a few other things), and won't have a Low power mode (not used that often), won't do some modes for as many hours as the DUO's, and won't have the built-in lid slots on the sides (tho I hear the lid can be stood up in the channel of the LUXes as well):
https://www.amazon.com/Instant-Pot-Multi-Use-Programmable-Stainless/dp/B01MFEBQH1 (this is also the latest "version" of the LUX 6 qt)

For more on PiP, check out my reply this morning to another person:
https://www.reddit.com/r/instantpot/comments/6mko8j/hurry_up_8_quart_duo_for_89_only_was_129_prime/dk31skq