Kev poked at it in his response so let me just come out and ask, what type of riding are you intending to do with your 700c (aka, 29") enduro/cyclocross tandem?

Next, what's your budget and what year is your RT1000?

Finally comes the question of your team's weight and riding style?
- Are you a combined weight of 270, 300, 340 or ????
- Do you plan to tour out on unimproved roads, are you looking to take on technical single track or something somewhere in between?

I can envision quite a few different scenarios and objectives, all of which have different solutions with regard to any modifications for your tandem. Dependent on the intended use, I could even envision that it would be better to sell the 700c C'Dale and opt for an MT1000, 2000 OR 3,000 -- which would be easier to made road-worthy vs turning the RT1000 into a mountain bike.