Can you play two different physical games at the same time with one Nintendo switch account?
Yes, the same user can be playing different games on different systems. I've tested this using Breath of the Wild, Mario Kart 8 Deluxe, and Pokemon Sword/Shield. You can even be playing different physical versions of the same game.
If there's a desire to not have save data be messed with when others play the switch, you can make a guest account for them to use.