What would you like to know?
I sailed the Gemini alot, so I have a pretty good idea of what works.
In general, put a 1-2 m2 smaller sail on the front, and the lighter weight
sailor on the front.
Use the centerboard only in non planing conditions.
Some of them have been snapped off by too big sails and too big sailors pushing
things way to hard.
The Gemini will go upwind quite nicely when planing, on the rear fin.
Again smaller sail on the front. Equal size sails and using the CB to go upwind
when planing puts excessive side forces on the centerboard.
What you will find is that when you first go out on the Gemini, after you stop laughing
at all the little "goofs" both of you make, and how "interesting and different it is to have to coordinate flipping; sails; ducking sails; (all the little coordination issues that both sailors must learn, real quickly) sailing a Gemini fast, is about the most fun you can have windsurfing.
If you have specific questions, I will be happy to try to answer them.
Basically it sails just like a single person board (you can sail it that way from the back
or middle mast slot by yourself), but if you put 2 sails on and 2 sailors, everything becomes like a dance with a partner. You have to decide when you are going to do transitions, and how you are going to do them, who does what/when.
You'll both fall in a few times, but then it will all come together and the fun begins.
The Gemini's a fairly fast (I've sailed them with 9.5/8.5; 9.5/7.5; 7.5/5.5 all the way down to a pair of 4.2 m2 Sailworks Huckers).
Just be aware that when a Gemini really gets up to speed it turns about as well as an aircraft carrier or supertanker, so leave yourself plenty of room for transitions when sailing with other sailors on smaller boards in big winds.
Hope this helps,