I've had good luck with the Maxxis Ardent 2.4 (EXO) tubeless on the mtb tandem. We tried the Panaracer Rampage 2.35 front and rear, but the rear tire tore a sidewall and we went back to riding that with a tube. Front tubeless has been good the entire year (well over 1000 miles).
The Ardent only comes in the 2.4 29inch tire with the EXO. For our purpose, the 2.4 is too wide to fit with the seatstay, I have yet to find a good rear tire with the tougher sidewalls. Until I do, I'll be using tubes on the rear tire. It seems that we are pretty tough on the sidewalls on the rear of the tandem.
For what its worth, I do like the Panaracer 2.35, I just wish it had a tougher sidewall...
Good luck and let us know what you figure out. You may want to post up on mtbr.com as there is a larger mtb tandem audience.