I forget to mention, just because two batteries are both AGM does not mean they are same. A starting battery and a deep cycle are physically constructed different, such that in some cases you will actually ruin a house battery trying to use it for a high current. The plates, the internal construction is not made to dump current in a hurry. Vice-versa, short, trickle loads cause starting batteries issues, they'll sulfinate faster, requiring maintenance charges and stuff to regain capacity.
But, OTOH, things like voltage will be fairly constant, so they can be charged in parallel as long as both are in good shape and there's sufficient current. Lead-acid batteries self regulate for the most part, so as their state of charge goes up, their internal resistance increases and current draw goes down. You would have more trouble potentially trying to do an AGM and flooded battery in parallel since they have slightly different voltage/current requirements.
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