Maybe these wiring diagrams will help......
Looks to me like your Inverter is designed to only provide one leg of 120V power. That means you need to choose which leg (and the circuits on that leg) on your Power Distribution Panel is going to get power from the Inverter.
I don't know that you could put the output into a subpanel and then branch off to the two legs of the Power Distribution Panel.
Perhaps Cavie or one of the other resident electricians will provide more knowledge and experience on this topic.
I did find this Inverter that appear to be able to handle two 120V inputs and two 120V outputs. I attached a screen shot from the manual for you as well.
https://www.etrailer.com/p-34275013.html
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