Can you really use 2 cables in place of /4 Romex?
No.

The language in 300.20(B) refers specifically to a single conductor, not a single cable. 

300.3(B) mentions "in a single cable" unless otherwise allowed, and I don't read 300.3(B)(3) or 300.20(B) as allowing otherwise.

I would guess the exceptions are intended for where you need to run a pair of conduits to physically accommodate large feeder wires; Which are "individual conductors."