[mindslight]

If you're talking about a workshop and anticipating that much ad-hoc power usage, I'd just put two dual 6-20 receptacles side by side rather than splitting one. And then since you're actually creating the premises wiring, stick an (L)14-20R next to them in parallel and get rid of your need to fuss with hacky combiner cords. At least that's what I plan to do when I have the time for such luxuries.

[bombela]

I only very rarely need 240V, if I had permanently mounted 240V machines or frequent needs, I would do exactly what you propose.