question

I think you have "Switch as a Group" checkbox selected within System Configurator. This means that both legs need to be present with 180° phase separation, as in a 50A service.
If you un-check "Switch as a Group" then it will allow one leg as in a 30A or 15A service.

Normally, split phase power is two legs 180 degrees apart. You may also find power that is 120 degrees between legs (two legs of three phase power). To accommodate this, select floating phase when configuring the system.

Even with floating phase selected when configuring for split phase operation, the system will reject shore power if the two legs are in phase as would be the case when using 30 amp 120 volt shore power and a 30 to 50 amp "dog bone" adapter. In this case, both legs are connected together and hence in phase.

You can make split phase power from 120 volt power with an autotransformer. But that adds space and weight and would require switching between the two power sources.

As wkirby suggets, you should be able to get one Multi to accept shore power by deselecting switch as a group when configuring the system. You may need to feed power to only one multi in this case. that is, connect the 30 amp 120 volt shore power to only one leg of your 50-amp plug to the trailer. Same would go for a generator if it is 120 volts and not split phase. In this mode, one Multi would accept shore power and pass through power to loads on it's leg. The other Multi would run in inverting mode. So the first Multi would power the second via the battery.