That's also what I did, but in reverse. Added the 100w portable(with its Viewstar) to existing 130w roof panel and Morningstar prostar30. But I couldn't see how the controllers would not see the input to batteries from each other and not read as battery state and get, at minimum, confused.
That and me reading 2 controllers which 'seemed' kinda silly.
So I removed the portables controller and ran to a plug from existing Controller. Mine is switched so I can see individual input and combined.
Works well.
I agree that the batteries can have multiple charge inputs and it works fine, its just the controllers are a bit smarter than gen or truck.