Multiple charge controllers on the same solar panel
Should work. Cheap charge controllers are a bang bang MOSFET switch, so either the battery is straight on the solar cell, or not. MPPT controllers have a switch mode buck converter; two of them on one panel will get very confused.

Once the battery is full, the charge controller switch will be mostly off, so the other charge controller can use the solar cell.

The danger might be that if both batteries are low, both controllers will come on together, shorting two batteries together and damaging the controllers. You could check for diodes or add them. They must go before the controllers, not between the controller and the battery, as it needs to sense the battery voltage.

The open questions are 1) are there already diodes in the charge controller, and 2) what does the controller do when the solar voltage collapses to ~14V (when the other one shorts the panel to the battery).  It might panic and not charge.
It's worth asking the supplier about both of these, I'm sure they've had these questions before