First-time poster on this sub-forum, and I want to say thanks for the information that's been posted here (and the willingness to share it).
On a leisurely metric on my single this weekend, we passed through some beautiful wetlands, a great blue heron took wing, and I thought, "It'd be nice if Joan [my wife] was here." and I naturally thought of tandems.... I'm an avid recreational bicyclist (C5 wannabe with beginners legs and 45+ masters age...), Joan less so, and I really think a tandem would be a great way for us to enjoy bicycling together in a way that two singles almost certainly won't. (Been there, done that.)
So, I perused this forum and some other sites on teh internets, and quickly decided "I want a Co-Motion Periscope Torpedo 700!" and decided almost as quickly that Joan and I will pedal straight to divorce court if I come home with a $3500 bicycle. (Ok, I haven't totally abandoned the idea of the Co-Motion, but it's not quite in striking range. So, let's move on, then, shall we? Don't be a tease.)
I will keep my eye on the used market, as I'm aware of the value to be had there, but in terms of new.... I suppose I can forget about steel (wah... my single is a Gunnar that I heart), which leads to the usual alu suspects - to wit:
KHS Milano @ $1650
Cannondale Road Tandem 3 @$1800
The beginner road bike Trek (T1000) is $2600, for reasons I don't really get, so that's kind of a non-starter.... The sub-$2k ceiling is fairly arbitrary, and while I'm a firm believer in "you get what you pay for," I also subscribe (reluctantly) to the little-known corollary: "how much do you need?"
Our needs, I imagine, are fairly minimal -- solo (well, duo) local rides, rides with clubs, weekends only (at least during the school year). Shorter distances to start, but I would guess that shortly we'll be doing 50 or so per weekend, working up to metric centuries and club rides and the like. No touring (supported or otherwise) is envisioned, nor are time trials, keeping up with the A racers, etc. We live in hilly east central MA (Bolton, Harvard....), and while there aren't mountains, per se, I will confess that I am drawn to disc brakes in order to avoid blowouts and related calamity from overheated rims. But I suppose those fears aren't especially warranted.
Of course, we're renting (from Wheelworks, in Belmont, MA) to see if we survive the experience (physically and emotionally), and they deal Trek and Co-Motion (the one card I'm holding out on the Co-Motion is its extreme adjustability... just the ticket for taking rides with my 11 and 13 year old sons when Joan decides she's rather... well, do just about anything than take another ride with me. I jest!), neither of which we're shopping for, so the purchase of the KHS or C'dale might very well come down to spec sheets, physical impressions, etc.
I'd like to get a solid beginner bike; actually, ideally, I'd like to get my second bike first (i.e., the Co-Motion), but I just can't rationalize (successfully, to significant others) the jump in cost. Yet.
Looking forward to someday posting on the Happy People on Their Tandems thread.
If you are a small to medium size couple, either bike will work just fine. If you are a larger couple, I would be more inclined to go with the Cannondale.
FWIW, you can find second hand KHS and C'dales for anywhere from $800 to $1,500 without looking too hard or long. The latter would be a good idea if you have already sewed the seeds for the Periscope. Frankly, with a couple of 11 and 13 year olds, your time on the tandem together will be short-lived so it would be best to focus on what will suit you and your wife.
Personally, I'd put aside any research on what to buy until you have spent enough time on the rental unit (you're a lucky couple to even have access to premium-level rental tandems) to either conclude tandeming is something that has captured your wife's interest or just a novel idea. Frankly, if she gets bitten by the bug the Co-Motion might be the right choice right out of the chute. Then again, if there's only luke warm interest keep renting or go the used route until you see a spark of greater interest.
I have ridden the KHS, and it is just fine. I ended up with a Co-Motion only because I found a deal, but was close with a few used Cannondales and Treks. You really won't go far wrong with any of these. I don't put much added value on the periscope.
I am in MA, pm me if you have local questions (I go to Wheelworks often, as it is near work, but I didn't get our tandem there).