Going through your manual, what I'm seeing is that each unit is single-phase, and when paralleled they can produce split-phase. You already know this of course. What the manual displays is that AC input is also split-phase. What MIGHT be possible is wiring a single-phase 120V generator only through L1 and N only? It appears that the manual never specifically mentions whether or not this is possible? Most likely you need to talk specifically with a Growatt engineer to find out if this is doable? Maybe perhaps it can work if one inverter is shut off, and it functions only as a single inverter putting out only 120VAC.
Perhaps one important question that needs to be asked is why you want to switch in the first place. I'm of the general inpression that split-phase generators are more or less of generally higher output, and maybe also generally better quality, so why is it important to you to switch from split-phase to single phase.
View attachment 117692