My partner and I originally owned a Santana Sovereign (aluminum) ... and currently own a Co-Motion Primera Co-Pilot (steel). We love both bikes for different reasons.

The Santana is lighter and we were probably 2+ mph faster overall. It was a medium/medium so too big for me (the stoker) and I wasn't able to use a shock absorbing seatpost, so the ride was uncomfortable. We did the National 24 Hour Challenge on it and completed 252 miles but I definitely had a sore rear.

The Co-Motion Primera is the company's entry bike. We bought this one on eBay for a great price because of the S&S couplers to take it apart and travel. It packs into two airline legal suitcases! We've made some upgrades to reduce its weight (new wheels, Gates carbon timing chain) but we struggle to hang with the single bikes on our training rides (reaching speeds of 27-28 mph). It's a small/small so I now can enjoy a Thudbuster seatpost. My captain is adjusting to having some toe overlap (especially when we're using the old wheels with the touring wider tires). The combination of steel, longer stoker cockpit, shock absorbing seatpost all make for a more comfortable, but slower, ride.

We are continuing to shop for a high end racing tandem. We were in Portland a few weeks ago and wanted to stop at that shop.