Suggestions:

1. Contact Santana to get an assessment based on the history of that year/model tandem and fork durability. My guess it, the fork's fine... Santana has always built it's frames and forks to endure a whole lot more punishment than folks of your size can dish out: think 450 lb teams, folks doing loaded tours and triplets.

2. If after Santana has reassured you that there were not systemic weaknesses in that year / model of fork you still want a fork that's stiffer, ask the folks at Santana what they recommend. They may have an inside track on 1" forks that are well-suited for your tandem. Be mindful of the cost relative to the fair market value of your current tandem (See #4, below).

3. If neither of those things trip-your-trigger, consider having someone like Steve Rex or Dennis Bushnell at R&E Cycles make a custom steel fork for your tandem. They can tailor it to your weight and set the rake to the stock Santana spec (55mm) or adjust it to suit your riding style, i.e., knock it down to something less than 55mm of rake such as Co-Motion's gold standard of 50mm or something even more racy; however, I wouldn't go much beyond 50mm for that year / model frame. Cost would most certainly be less than a tandem-specific carbon model and while it probably won't be as light as a carbon fork, it will most likely be plenty stiff and something less than 900 grams given your team weight.

4. If that doesn't scratch your itch, perhaps it's really time to start looking for a new used tandem or perhaps even a new frame/fork given the amount of money that you'll end up sinking into the old tandem. Y'all seem to ride the wheels off of that thing based on Pete's blog and tandems have come a long way since that one was built. You could probably cover more than 1/2 the cost of a new tandem by selling your old one... but only after you have the new to you tandem so that you can trade out any chi-chi parts you'd like to keep, e.g., the Campy stuff and daVinci cranks, etc.). Seriously, if you two are hooked on tandems and riding that thing aggressively in challenging terrain and have an inclination towards doing time trials or other major challenges, the acquisition of a contemporary frame will be money well spent. Probably a good time to do some cost modelling of those different options.