RE: surfing the gemini
Sailing the Gemini (or any tandem board for that matter) is about the most fun you can have on the water, but it takes a real "let's give it a try" attitude and you have to be able to laugh at yourselves alot.
FRont sail can "duck jibe OK, so can the rear, but getting that coordinated would be a real laugh riot as the sailor in the front would have to perfect "duck jibing in a vacuum" first.
REally, it's pretty easy, you just carve up into your tacks with the power in the rear sail, the sailor in front gets his rig over and draws the nose down and then the rear sailor sheets in and off you go.
The first few times you do this, can be real interesting.
If the board doesn't make it through the eye of the wind, you often end up "in irons" and backing up. Kinda hard to get the nose to bear off when the board is gaining way straight backwards, so you laugh, use the rigs to turn the board, and try again.
Jibing is easier (perhaps a little more dangerous for the front sailor) as the front sailor gets to flip the sail in a complete windshadow (or maybe something more like a moving wind shadow) so getting the front rig around is easy.
The rear sailor hangs on clew firstand tells the front sailor the clew is coming across so the front sailor can duck as the clew comes round.
After you get hit on the head a couple of times and you both laugh out loud about it, the coordination begins to develop and the front sailor doesn't need a helmet anymore.
Lot's of fun, and big laughs.
I've benn both on the front and the back of the Gemini with 2 4.8 m2 Sailworks Huckers on it and I wouldn't let the pilot go any faster than 25 knots (on his wrist mounted GPS.
I guess Antoine and Julian have had the Gemini up to almost 35 knots!
That's scary fast!
If something (or someone) comes apart at that speed, 2 rigs crashing and thrashing is going to make a bit of a train wreck.
Hope this helps,