We did this this year with our vex team. It worked well for recruiting more girls, as we went from 3 to about 9. However, make sure that you have the funds to support this team. Add up all the hidden costs of a new team that you may not have thought about in a while, such as new crate, new cart, new shirts, and then double all recurring yearly expenses. Assume on more mentors. Finally, assume on doubling the quantities of tools that are always in use (try building two vex robots with only one allen set
) If you can pass this test comforitably, and have nothing else like "we should really be saving for a cnc lathe" planned, then go for it.
If you do, i suggest that you keep as many activities outside of construction of the robot as together as possible. Judging by the numbers you have posted, and assuming that there will be a large effort to get more girls to join, I am assuming that the girl team will have less experience. This happened with our team, however, there was an initail effort to "let the girls do it on their own." However, midway through the build, ideas began flowing between teams, and both the teams were much better off. We would have been even better off if we had held design sessions, etc. together. If the teams are mentoring each other, it both helps both teams, and pays off in the chairman's submission.
Good luck in your descision.
PS. see
this thread on a similar topic when my team was going through the same thing.