Using 12/3 non-metallic cable to carry 2 circuits
This is a MWBC - a multi-wire branch circuit. It is perfectly fine, provided that:


The two circuits are on different legs - i.e., 240V between the two hots. If so, the neutral is the difference between the circuits instead of the sum of the circuits. In other words, if you have 15A on one circuit and 10A on the other, the neutral will actually have 15 - 10 = 5A, not 25A.
The two circuits have common shutoff - i.e., if you turn off the breaker for one circuit, the breaker for the other circuit always turns off at the same time. That is important because otherwise the shared neutral could have current on it from the other circuit when you think everything is off because the breaker for one circuit is off.


There are also complications when it comes to wiring up GFCI on an MWBC, but if installed correctly an MWBC is perfectly safe. Depending on the configuration, you may want to install a dual-pole GFCI in the panel (advantage: one location and "done"; disadvantage: long way to go to reset) or you may want to install receptacle/GFCI devices at the point of use (advantage: easy reset when needed; disadvantage: depending on how/where the MWBC is split, this may not be practical or may require other changes to the wiring).