I have.
In about 1980 I put an ad in the local paper asking for free bikes. I got loads of replys and collected a two gents bikes and a woman's frame. I put the biggest one at the front, the next in the middle and the woman's frame at the back. I cut the chain and seat stays off the front bike then sawed vertically through the head tube of the next frame. Repeat for back frame. To keep the frames in alignment, I got some 4×2 and laid out on a flat surface put the frames on top, put more 4×2 on top and weighed it all down with concrete blocks. G-clamped the frames together – the cut head tube butting up to the seat tube of the frame in front. Then I cut up an old garden swing and used the legs to connect the bottom brackets. Then I brazed the whole lot together using an arc welder with carbon rods.
For the drive train I used 10 speed front chainsets on the right hand side of every frame. From the front, the inner ring was connected to the inner ring on the middle frame and the outer ring of the middle frame was connected to the outer ring of the rear frame. The inner ring of the rear frame was connected to the rear hub. I used frame mounted (bodged) tensioners to keep the chains tight.
Rear rider had control of gears and rear brake! Driver only had steering and front brake.
The biggest problem was that it wasn't nearly stiff enough as I didn't have access to suitable tubing and probably because I brazed instead of welded. Frame flex eventually became terminal but not before I offer a lift home to a drunken mate on New Year's Eve failing to mention that we had to cycle home 7 miles on the Trandem.
No pics as only have film photo somewhere but it was good fun.
Good luck