When I built mine back in the late 80's we had 2 big race sails to start with (6.2 and 7.0) with the bigger one at the rear - we were both around 70-75kgs. To be honest, didn't do too much mucking around with the sails - we were stoked just to have the thing float!!
In more recent years we have been using wave sails (5.0-5.3) but only get it out over 20 knots now. At the front we put the heavier sailor and bigger sail. The reasons being - it was easier to gybe (rear guy had to duck gybes) and when on the plane the sails felt as one (full foil).
The front guy hardly did anything - just tweaked the sail as required, watched out for obstacles and gave directions when necessary. The rear guy did most of the board adjustments.
A couple of pointers if I may...
1. Chose your partner wisely - they need to be roughly the same skill level as it will make it easier.
2. Be prepared to fall in a lot - mainly from laughing so hard
3. Waterstarts - the front guy needs to be almost fully up before the rear one starts to get up. we could uphaul it as well but this was harder for some reason
4. If you are the front sailor - watch out for catapults - if you hear a scream then a splash from the rear sailor - it is already too late - you will stop very very suddenly
5. Give yourselves a lot of room to gybe - it takes a bloody big arc and time to complete.
6. Do not attempt to jump waves! We took it in the surf once and besides the massive flex in the board, the front sailor got a nose bleed from being 6 foot off the top of the wave before the rear sailor had even hit the ramp!
7. Get video - it is awesome to look back at
8. Have fun

I have a couple of photos and video I'll try to locate over the weekend - enjoy your trial and error!!

Chubb

Quick reply (winds up!) More later.

We have speed tandem and also Gemini Tandem. (Slalom Tandem in progress)

Speed tandem only sailable on PLUS 25 knots and preferably High 20's. Always used slightly larger sail at the back with two 70-75 KG sailors. With 80 KG sailor on the front used same sailed sails, but this is not the normal.

Cant gybe speed board. Just too cramped for room. Hard to water start. Found that if the front guy gets up too early, the board speed drags the back guy out of position. Timing critical.

Critical that both sailors don't try to foot steer and work against each other to get speed wobbles. Also front sailors can't sheet in too far as it kills power in rear sail. mast foot position is critical and Fin position is critical for directional trim. (A Box adjustable)

This run did 35 knots in 2006/7 - Two 5.8m sails. 75 KG on back, 80 KG on front.



These runs done with two 70 KG sailors in 1991. Sails - 2,9m on front. 3.5m on back (35-40 Knots)





Gemini Tandem very different. We usually only sail it in light winds with big sails. 7m to 9m. Does not matter much if one sailor is slightly bigger than the other but if big difference we usually have the larger one on the rear with the larger sail. Always try to have larger sail on the rear. Speed limited with race sails as to how far back can get mast tracks before dragging foot of sails in the water. Usually top speed around 20 knots, d Did 25 knots with two 5.8m sails but very unbalanced.

Here is 75 KG sailor front. 75 KG rear. 7.5m front, 9m rear. 10-11 knots of wind.




More later. :-)