I've tried to teach students using the Gemini last year (2007 when I had a Gemini in the demo fleet) and I would agree, it sort of works.
On the plus side, you can expose a student to faster sailing (even planing) by using a large rig for the instructor on the rear rig position, and this will get many students "stoked" on windsurfing far earlier in the learning curve than any other method I'm aware of.
On the downside, the student does not really get the full picture of what they need to do as the instructor is really sailing the board with the larger rig at the back.
Also, since the instructor is behind the student (you cannot very well sail the Gemini from the front position) the student cannot really see what the instructor is doing.
I've found the "between the boards tether system" (I think you saw me use this the last couple of years at Lake Okabena) works far better to get the students learning the fastest.
1/ They can see (and imitate) the things the instructor is doing right in front of them.
2/ They are on their own board with their own rig, so mistakes show up right away.
This gives them a much better feel for what is needed to sail the board themselves.
3/ As soon as the instructor sees that the student can sail along, tack, stay upwind,
etc. on their own, the tether is removed and the instructor can coach them by sailing close by. This helps the student become confident in their own skills and abilities very quickly.
The whole process normally takes less that 15 min. to get the student to the point of sailing their own board, controlling their own direction and working with their own rig.
Nothing else I've tried has worked as quickly or as well.
Yes, the student still needs the same amount of simulator time as the time on the simulator is where they learn basic rig handling skills, uphauling, how to tack and jibe,
and the basics of controlling the power in the rig and getting the board to go in the direction the want to go.
Hope this helps,