To be completely safe, you might want to fully charge them
individually before you connect them together. If you connect two
batteries with considerably differing charge state, one battery will
charge the other at an uncontrolled current.
--
RoRo


Or place a Schottky diode in series with each charge port, along with the
series fuse that someone else suggested, making sure of course that the
charge source has sufficient voltage overhead to still allow the battery to
reach a full charge voltage level after taking the diode drop into
consideration. That way, no battery that's connected can become a charge
current source for any other, no matter what their relative charge states
are.

Arfa