When a circuit is complete, the neutral line carries current. If you were to fully load one of the circuits (originally fed by the black wire) with 20 amps, and, at the same time, you were to load the other circuit (originally fed by the red) with 20 amps, you would have 40 amps returning on the single neutral (at least from the crawlspace junction box back to the panel). While there is a good deal of over-engineering built into the code, I think you are creating a risk of overloading that neutral line (basically an overheating risk, but perhaps a voltage drop issue as well). I am not sure, but I think the breaker only senses the load on the hot leg, and that overload would not trip the 20+ draw traveling over the neutral.