It's a very do-able modification given the conditions you describe but not necessarily inexpensive; perhaps as much as $900 - $1k if you don't already own a heavy-duty set of disc compatible 26" wheels. The brakes are $100/ea and the fork can be made for a reasonable amount of money but a good set of disc wheels for a tandem can run any where from $350 - $600 depending on your hub selection and any bro-deals you might be able to get.
However, I think I'd stop my modifications at the disc-compatible fork, 26" wheels, disc brakes and stick with the STI drop-bar / cyclocross configuration. You'd also want to considering upsizing your chain rings if you want to keep your current net gearing, i.e., a 53t chain ring on a 700c wheel = about a 56t chain ring on a 26" wheel, 42t = etc...
I was curious regarding model year to be sure you had the rear disc brake fitting. The use of a rigid fork modified for use with disc brakes is very do-able as well. However, I would contact someone like Glenn Erickson of Erickson Cycles (GlennEBike@aol.com), Todd Shusterman at daVinci Designs (email@example.com) or perhaps Dennis Bushnell of Bushnell Cycle Designs (firstname.lastname@example.org) who works closely with Glenn Erickson regarding your need for a custom-made, disc-brake compatible 700c fork. I don't think it would cost more than $200 - $250max, but you need somone who is familiar with the use of front-mounted disc brakes for tandems. Glenn & Todd are the only two builders whose forks I've seen that address a safety issue with how the front disc brakes are mounted and I suspect Dennis has adopted it was well. It's not a high-tech fix, but it makes all the difference in the world with regard to safety. Let them know what you're trying to do and they should be able to advise you on if and how to proceed.
Back to your handlebar system, here's the problem -- cable pull. I'd assume you're interested in using Avid's Ball Bearing disc brakes since they are at present about the easiest and most reliable. The Avid discs come in two different models -- MTB and Road -- because MTB & STI/Road brake levers have different amounts of cable pull and therein lies the problem with your bar switching. While there are ways to jury-rig MTB brakes to work with STI levers (cable-boosters like Travel Agents) and to jury-rig Road brakes to work with MTB levers (excess cable run-out and slack asjustments), neither are optimal set-ups. So, I'd really discourage the bar-changing scenario and recommend you go with one or the other, i.e., MTB bars & levers or STI. The ONLY other option would be to ditch your STI and use bar-end shifters with a specific model of DiaComp road brake lever that uses the same amount of cable pull as an MTB lever.
The cable splitters you are thinking of are called "Easy-Split" In-Line Cable Separators and are made by daVinci Design of Colorado. They cost about $11/ea and you'd need 2 of the derailleur cable models and two of the brake cable models. Tandem-speciality dealers like Gear-to-Go Tandems, Tandems East, Tandems Limited and Precision Tandems usually have them in stock.
Hope this helps and doesn't make your head hurt too much.
Last edited by livngood; 07-15-03 at 11:56 AM.