Reason: They are both in use and seem to be interchangable in practice. Unless there is a good reason to distinguish between one and two hands, they should be aliased. If we don't alias, it should still be cleaned up.
They seem fairly different in what they're trying to tag to me. hand on hip as the name suggests is just one hand on the hip, while hands on hips seems to be really specifically arms akimbo for the most part. Around 41 of the 63 hands on hips images depict arms akimbo, the remainder then coincides with what hand on hip was tagging or depict a person holding the hips of another person they're humping.
As for the word akimbo, most of the dictionaries I'm coming across seem to indicate having both hands on the hips.
Maybe alias hands_on_hips to arms_akimbo then to make it more obviously semantically seperated? I'd even suggest plain akimbo if it wasn't already claimed by the FPS usage.