A male/female pair should be fine in a tank that size. Provide appropriate foliage/cover and they should get along great. Male female aggression can happen but is not nearly as common as male/male aggression so make sure the pair is properly sexed as it can be a bit tricky with the small Phelsuma spp.

Although a pair should be fine, keep a close eye on them for problems and separate them if things get out of hand.

good luck
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Zach