I don't have either one, and have not spent time on either.

While we do have an older Cannondale MTB tandem, I will say it is a good performer.

While shopping for a road tandem, I did a bit of homework, both on older bikes and newer ones.

You will find if you look at the specs, that for 2008 and prior street tandems they were based off the mountain frame.

For 2009 Cannondale retired the older Road Tandem and Street Tandem frame design, plus they currently do not offer a US mountain tandem.

What they did appear to do though is utilize the basic 2009 Road Tandem design and geometry for all the bikes. The difference being the 2009 Street Tandem sports 26" wheels with 2.1" tires. I'm sure they needed to widen the seatstays slightly on the Street Tandem to fit the 2.1 tire, but other than that the frames sure appear very close to the same.

Honestly to answer your asking about the largest tire in an RT3 frame, I don't know.

Obviously these 2.1" multi purpose tires could be refitted with small profile 26" slicks if needed. Also if you needed to run 700c wheels, these would probably fit but would require converting to discs as I doubt the V brakes would line up.

If you opt away from the 2009 RT3, you might do better with the 2009 Street Tandem.

If you need a real off road bike, you might need two like some of have had to do.

Hope it helped.

PK