Luminous Optimus 2800 has a mode setting. By default/factory it's set to Eco mode (to save power they claim but it's just slower switching) you can switch it to UPS mode for fast switching. I had the same issue with the one I bought locally, replaced it ASAP got the second one, which had some transport damage then replaced it again to get the unit I have right now only to RTFM and find out it works in correct mode.
And no the batteries are in series so at 2000W same amount of current goes through both single and double battery setup. High power inverter have multi battery setup cuz it's logical that if you want more power, you prob want to store more of it.
Inverter is very crucial at my farm. We had been using single battery setup for last 10 years, got multiple inverters in that time, highest capacity we could get but it was never satisfactory. Switched to 150x2 setup and it's magic. I wanted to go with 200x2 but the inverter guy explained that keeping a higher capacity battery at 80-100% is difficult. You get a longer lifespan with 150x2.
Note: On phone ignore brevity
