No. They would take whatever charge they are capable of holding... HOWEVER, in a nutshell, they will only supply power at the rate of the weakest battery. So if one battery is about dead, the other one will act like it is too. If you are camping at the moment and need battery power, just take the bad one out of the loop and use the remaining good cell by itself.
That is.. if you're talking two 12v's vs two 6v's.
Just a note: When changing batteries it is important to change all batteries at the same time with the same type, age, and capacity.