Unfortunately (for you), I'm with Dean on this. 20 lb is not realistically achievable in a rideable/reliable tandem. I built up my Calfee Tetra (about 200g more than the claimed Santana) with a budget but light build and hit 25 lb. While there are a number of places I could have saved weight, it was all small amounts per part, usually for large differences in price. I could likely have kept the build in the 23-24 range for an extra $1000-2000, but even spending another $5k (my total parts list aside from frame and fork was under $3500) wouldn't have gotten me anywhere near 20 lb.