Yes, i want to monitor and charge the 2 batteries individually. The idea of the transistors it to be able to cut the solar panel so that the batteries don't get overcharged.

The "up to 5v" module does regulate itself to step up or down keeping the output voltage

Should I try putting one more diode to link the solar panel and the 5v module so that if the batteries are charged it can draw power directly from the solar panel or a simple diode wouldn't be enough?

And yes, i've used a similar calculator - the 1k/1k resistors should cut the voltage in half and as the solar panel does not go above 8v we should be fine, right?

 

All you really need is one diode to make sure the batteries are not powering the solar pane; when there is no sun.

 

Don't the transistors stop the back current?