Another update:
I did get the twin 3048LV-MKs paralleled. They do output 240V when configured that way, both via inverting from the battery and being fed 120V input. I don't have an easy way to test their load capacity in this configuration at the moment, but, meter checks do show 240V output on the lugs.
At this point, I discovered a fairly significant snag: when paralleled to output 240V split phase, you cannot charge the battery via 120V input. This was exceptionally frustrating to discover, as, I do need utility or generator input to charge the batteries in bad weather or if the grid is down. This is a big deal that isn't at all mentioned in any documentation, and I only infered that this was the situation due to watching one of Ian's videos on the bigger brother 60-something AIO that's got two internal 3kW inverters. I had watched that video at least two times before, but it didn't register that he had to apply 240V input in order to get that unit to charge the batteries via utility when configured to run at 240V output.
I read as much as I could and determined that the best solution was simply to get a Victron autotransformer, which I did, and I installed for testing, and it works perfectly.
Annoyingly, had I known about this limitation, I would have likely picked a different setup, but, as it is, it's fine and also affords a degree of flexibility that I would not have otherwise. My thinking is that if I don't need the extra output of the second 3048LV-MK, I can just use the autotransformer to create 240V from one inverter and save the running wattage of the second inverter. 3000W is more than enough for all my needs as long as I don't need to run a minisplit at full tilt and a high draw appliance like my electric kettle at the same time, at least from battery and not utilizing grid pass through.
Still, for now, I think I'll install it as I've got it now, with the autotransformer feeding both 3048LV-MKs as that's more convenient in some ways for wiring the whole thing.
As usual, one step at a time.