Using 2 Separate NM Cables to Make 1 4-wire conductor?
Nope -- this violates the intent of 300.3(B)/300.20(B)

Trying to "split" a circuit's conductors among cables or raceways is generally prohibited by NEC 300.3(B), with a few exceptions (that are not applicable to typical residential wiring, even if they appear so at first glance).

The issue is that if you have current going out to the load on one wire and back on another, the resulting loop generates a magnetic field within it.  This is a EMI hazard as it can induce 60Hz noise into sensitive devices (old tube TVs and monitors were the most common examples), but more importantly, it's a fire hazard because that magnetic field can go on to induce eddy currents into ferrous objects within that loop, heating them up just like a pan on an induction cooktop.

As a result, you're better off running a new 6/3 cable for the new oven, as sadly, there is no such thing as a 240V-only wall oven that I have found listed for US use, and the old-style 3-wire connections are prohibited for new wiring for very good reason.