I have installed a number of receptacles around my woodworking shop, all NEMA 5-20R duplex receptacles with separate feeds to their top and bottom halves, and a shared neutral and ground. These are all behind a single common-trip two-pole 20A breaker (GCFI), using 12-3/wg Romex to "daisy chain" from one duplex receptacle to the next (two hots from alternate legs, neutral, and ground).
I now find myself needing a 20A 240V receptacle in one of these locations. I am not worried about overloading, as no other tools will ever be operating on the circuit when this one is in use.
Can I replace one of these duplex NEMA 5-20R with a single NEMA 6-20R (alternating hots plus ground), and if so, does the neutral-to-neutral to continue the chain count as a "splice" in my box-fill calculations?
I think that the common-trip breaker exempts me from the "only line-to-neutral" requirement (2020 NEC 210.4.C), and cannot seem to find any other restrictions about mixing line-to-line (240V) and line-to-neutral (120V) receptacles.