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Hi again coolman. This won't be an oversight, but 'pennypinching' is too harsh. Victron have an incredible range of kit for all buyer levels, and the bigger/better you go, the more features you can expect. And a higher price. Such is life.

Your VenusGX has 2x VEBus ports, so all is not lost. Yes, it's a 'bus', so should work fine to add another Multi there.

Note you may strike issues adding another Multi in parallel if it doesn't match in hardware/size into a common ac load. There's a thread about here somewhere that discusses that. Not to say you can't add another Multi easily into separate ac circuits.

If you're imagining 3 phase in your future, please try not to. A quantum step up for a smallish offgridder, usually unnecessary, and best avoided. Like the plague..

Splitters are indeed required.

I think its amazing that these tiny and affordable units even have the parallel and multiphase features!

Back-wiring can help with neatness.

Please read this parallel and three-phase manual very carefully. There are limitations and restrictions with these more advanced configurations, it is STRONGLY recommended get the units at once so that they are compatible.