Will 12/2 fit in 3/4 conduit?

One 12/2 Romex fits in three quarter inch conduit, and that is the honest capacity. Flat cable counts at its widest dimension as if it were round, per NEC Chapter 9 Note 9, and a 12/2's wide profile eats most of a 3/4 pipe's allowable fill.

Two 12/2s in one 3/4 run violates fill and, worse, makes the pull miserable enough that the jackets scrape.

Sleeving one cable for physical protection is exactly what this arrangement is for.

Where multiple circuits share a raceway, individual THWN conductors are the right tool: the same 3/4 EMT that fits one Romex takes sixteen number 12 THHN wires.

The PVC conduit fill calculator handles cable-in-conduit fill with the cable dimensions built in.