Our team weight is less that 350# and half of us are not power riders.
Our first tandem was a steel Kent in red. which we picked up for about $300.00. I sold that one within a week and picked up a road Giordano Viaggio aluminum with the expectation of having to replace a lot of crappy components. It is a very harsh riding aluminum frame. With new quality drive train components, it didn't ride as easy as the Kent.
We are replacing the Giodanno frame and I would not even consider with a Ti frame. I've seen too many anecdotal stories of it developing stress cracks and on a tandem wear and strain is compounded. For your team weight, my complete non professional opinion, is to go with a steel frame. My road bike is carbon and rides well; however, it is quite chattery on rougher roads.
We have an eye possibly on a Ritchey or I will go back to Doug Curtiss at Curtlo cycles for a custom build. Doug built our mountain tandem for us and did a FANTASTIC job through the entire process.