Combining a Multiwire Branch Circuit on a single breaker
Since


the two hot conductors do not rejoin at any point (we know this because you'd have a 240V short on your hands otherwise), making it so that the rules about wires in parallel don't apply
the two hot conductors are being protected by the same breaker in the new configuration (i.e. two 14AWG hots on the same 15A breaker, returning via a 14AWG neutral), eliminating the possibility of an overloaded neutral, and
this is all on one breaker, making it all part of one branch circuit and avoiding a violation of 200.4(A) regarding shared neutrals


you can go ahead and do this safely by wirenutting both hots to a pigtail from the chosen breaker in the panel.  (312.8 allows splicing in the panel provided you aren't overflowing the gutter space.)

I would put a tag on the wires in the cable saying "Same Leg" at the various boxes this feeds though to avoid confusion between this (admittedly somewhat odd) configuration and an actual multi-wire branch circuit.