OK, so in the series split-phase configuration, if the inverter manufacturer insists on measuring voltage and interpreting configuration settings with the - probe at neutral, I feel we should follow their terminology and call one of those 0 degrees and one of those 180 degrees. (EG V1 = L1 - N, V2 = L2 - N). Otherwise stuff will not work. (I've been trying to find an example EG from victron on how they interpret this, but I was not able to in the allotted time).
If the inverter manufacturer insists on measuring / interpreting the settings the same direction on both legs (EG V1 = L1 - N, V2 = N - L2) then it should be 0 degrees.
What would you call (series or parallel) a split-phase inverter that can be configured in either L1/L2/N, or L1/L1/N (120V) mode, without changing any wiring inside the inverter (and it may not even be possible to change if the neutrals are bonded together on a busbar and can't be swapped). I'd prefer to refer to one of the inverters as configured in 0 degree mode for 120V and 180 degree mode for split phase because we have not done any physical rearranging of how the inverters are wired up.