I know titanium frames can be made so light they ride like wet spaghetti, and I know design is important for stiffness, but what might be the minimum weight for a rideable frame. I'm finding some in the 2.5-3.5kg range...Thoughts?

Spend your time looking for builders with a good reputation, talk with them about what you're looking for and then talk to a couple of owners to see if the builder met their expectations. That's far more important than the weight of a frame, given most of the high-end tandem frames all fall in the 5.5 - 8 lb range, regardless of material, e.g., Carbon, titanium, Magnesium, aluminum and even a few steel frames. Put another way, it's the difference between a 26 lb tandem and a 28.5 lb tandem where four water bottles add nearly 6 lbs to the total bike weight.

A reference point is Seven cycles. Per web site below 7.7 lbs ave frame weight.


https://www.sevencycles.com/discipline/road-tandem.php

The Seven site lists the Ti tandem frame at 7.7 lbs. and the steel tandem frame at 8.9 lbs. As a frame gets larger and stiffer the weight difference becomes less because of the nature of the two metals.