Deriving a 220v source from two 120V receptacles

[kwired]

My "objection" is in reference to obtaining the line-to-line power from two separate circuits and outlets.

If this was a 3-wire MWBC to one box, then the new wiring would be merely an extension of the circuit.

Once a MWBC becomes separate 2-wire extensions, I don't see feeding a single 3-wire load from them.

I think it is a good idea to keep it all in same raceway/cable, but at same time NEC specifically mentions non ferrous wiring methods in 300.3(B)(3), so you can't object to it on code if that is what is done, but rather object to it based on other design principles.