Oops - did not see his existing 2K Multiplus on my tablet....
Multis can parallel up with each other. They also parallel in a similar fashion with a genset or AC mains much like paralleling two compatible gensets. The manual indicates the Multi needs a minute or so to synch up its waveform with the genset. The question is how much current can they handle? From what I can tell, the 2K multi has a 50A passthru transfer switch. It itself has a 4k peak output which it would add to the genset output. If the generator is large enough, the multi does not get into the mix - its a passthru transfer - any excess AC mains input would be diverted to charging the batteries.
See:
https://www.victronenergy.com/inverters-chargers/multiplus-12v-24v-48v-800va-3kva#whitepapers
From
https://www.victronenergy.com/upload/documents/White-paper-Achieving-the-impossible-EN.pdf on the Victron Site:
"Sometimes the features of a new product are so unique that the benefits are difficult to understand or simply denied. The Phoenix MultiPlus is
such a product.
The purpose of this paper is to step by step clarify the many features of the Phoenix Multi/MultiPlus.
Think of a situation where electricity from the grid or a generator is not available or limited in power. Think of boats, mobile homes, trucks,
service vans, remote communication systems or off-grid houses. All too often shore power in the marina or on the camping ground is limited. All
too often small gensets are a headache because of premature failure or frequent maintenance.
The Phoenix Multi/MultiPlus solves these problems.
1.1 The bidirectional converter
The M/M+ is built around a bidirectional converter, that operates as an inverter or as a battery charger.
The converter of the M+ does more than a conventional bidirectional converter: it operates in parallel with the grid, with shore power or
with a genset. It can add power to AC supply (with energy from the battery) and it will use surplus power to recharge. In most cases the M+
will also reduce the harmonic distortion of the genset voltage (this can be important for certain sensitive loads such as an induction cooker)
Achieving the impossible-Taking control of shore power with the Phoenix Multi/MultiPlus fromVictron Energy"