Use One Solar Panel to Charge Two Small, Identical Batteries
These things are the dumbest solar charge circuits possible. See a tear down here 
https://youtu.be/OSR2ofEAJKg

Solar cell to reverse protection diode to 6x series AA NiCD battery pack. No charge controller. It depends on the battery pack getting trickled charged, and being stronger than the solar panel.

Connecting the same panel to two lights will cut the current to both, and likely tripling the charge time (it won't be exactly half). The built in diodes on the input board will prevent one battery from charging the other, just as they prevent the battery from back feeding the solar panel in normal use. No modification or precautions needed.

These lights will already deep discharge the batteries over night as is, which is why NiCD are chosen. Their runtime will be reduced due to less power charging them.

FYI you can buy similar panels from overseas online for a few bucks. Probably from the same place HF gets theirs.