Personally, I would also consider a TOTAL rebuild of your T900, keeping just the frame and maybe the fork. Price all of your options. Come up with a MUST have and a WANT list, then map your options against those.
IF changing, I would look for a 559 rimmed tandem that is capable of accepting 58-622 tires (larger OD than 28-622) which will soak up the bumps and roll well.