Everyone who has posted here has been right...picking a house is very personal and rush is about mutual selection, so it's not like you can both go in and agree to join a certain house, b/c you only have so much control over the situation. And unless you are in the same rush group, you won't be able to communicate when it's time to do your rankings...and if you are in the same rush group, you shouldn't be discussing that stuff, anyway..."tent talking."
Just reassure your friend that you'll love her no matter what houses you guys end up in, and then if you do end up joining different houses, it could be fun...
You could go shopping for socials together, do each other's hair for your formals, if you live in the house you can "escape" to her house for a breather...this could add a new dimension to your friendship.
Good luck with Recruitment!