Hi there,
It sounds like you girls are going to have some fun. I am a female dancer and I'v thought about what happends the da I do a showcase with another girl. It will happen one day, because sometimes we have couple of the same sex comming to the studio to take dance lessons.
If I get to choose it will be a combination of rumba and cha cha. In the rumba section do not be afraid of bodycontact and in the cha cha section have a lot of fun. I'll promise you that the audience will gasp and be thrilled. Remember you are both artist and want to tell a story.
I think two girls i smart dresses dancing together in an dynamic take-and-give sort of way, must be so good to watch. Be sensual and dear to be sexy and professional in the same expresion.
Move like a couple, dance like a couple, because you do not want a hip hop routine. Let one be the dominate and then switch roles into the song. Maybe one of you can drop or lift the other at the end?
Side by side is fine, but wath about chasing eachother in a cha cha? I know it might sounds a little akward, but we live in a modern time and try something new. I'm sure you are both good dancers and want to make the best out of it.
Good luck, I would love to hear how you did.