Don -
The easist way is to put a diode in series with each battery, to isolate them. In other words, neither battery can try to backfeed the other, and the charging source will see the lower of the two batteries and will charge that one until they are equal. This is not a perfect solution because the diode will cause a voltage drop in each charge line, and the charging source will see the sum of the battery voltage plus the diode drop. However, if you use a Schottky diode (as opposed to a silicon rectifier diode), the drop will be less than 1/4 volt, as opposed to the 0.6 or 0.7 volts of a silicon rectifier.
There are other, more elgant solutions, but I would try this first.
Bill
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