I like the idea, though I'm not sure the lawful/chaotic opposites would work well together, even for a small bit of time.
Other options could be Ardad Lili, one of the whore queens of Hell, perhaps helping Asmodeus or replacing him as the main deitific creature of focus that, if she can subsume the world, may become a true lord of Hell or some such.
Alternatively, if Mirta is the made up god of the realm...could just make up an evil deity that has sex as a focus.