I wouldn’t bother to turn off the 500VA.
If the big Multi is running, that means there’s a big load and the self-consumption of the 500VA won’t matter that much.
But then question remains how you’re going to switch on the big Multi.
Manually or automatic ?
We are basically a DC ship for all essentials - Shoreline R134a converted domestic under the counter fridge from UK, keel cooled custom built frigoboat chest freezer under the galley deck, water pump, sump pumps, lighting, satellite receiver etc etc. So when we use AC, we turn on the MP3000 from a number of remote switches plus a timer.
The fridge stopped cooling - gas pressure fine but just not enough refridgerant getting into the evap - probably partially blocked capilliary tube. French fridge engineers won’t service R134a anymore and especially on a boat.
So new fridge crisis purchase. Vitrifrigo, Frigoboat, Dometic etc didn’t make anything suitable and/or €€€€. So a Liebherr 230v and the 24/500.
In that case you could just hook the Liebherr fridge to the 24/500 and call it a day ?
As I’m currently doing. But nice to run some very low wattage items eg ceiling fans off the smaller inverter overnight rather than just heating air from the MP3000. 
I got a different type of ATS, which has 2 sets of NC and NO contacts, which would operate the 500 remote switching. It works great with the 3000 simulating the change over. But the 500VA doesn’t have the oomph and trips out on a low AC voltage warning. The load seen in Connect isn’t into the red zone. I guess the switch over coil pulls the AC voltage down too quickly. I guess I could try at 245v?
245v didn’t make any difference. My ATS is 100A, so perhaps the coils require greater oomph and a 25A capability would better match my max paired MP3000s.
Just measured a coil - only 18 ohms so 12A plus surge. No wonder the 500 couldn’t shift it.
I must find out if lower current rated switches have coils with more resistance/less current draw.
That could be an issue indeed.
For example, a Schneider Electric LC1D95P7 has a coil rated at 245VA inrush and 26VA holding.
Have you tried lowering the output voltage? Since higher voltage means a fixed resistance will draw more current. Its a bit complexer with coils/inductive loads, but a also there, in general a lower voltage will reduce the power draw
I’ve reverted to my original design in post 6. Once the 500’s CN11 contactor is made, the Victron Connect shows a base 6VA/W load - same as with nothing connected.