You should not do this because the two batteries will most likely be at different states of charge and so the battery with the highest state of charge will exceed 4.2V per cell and will be over charged. If you had the charger in balance mode it should prevent this but I do not advise it due to the severe imbalance that the charger would have to cater for.
IF the two batteries are used in a 1/8th car where you are running it at 14.4V all of the time and the two batteries are always used together all of the time then you could charge them in series on balance mode because the two batteries will have the same state of charge... but this would be the only exception that I can think of.
Neil
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