Interesting problem: Key questions are as Zona said,
1) How much you want to / have to spend?
2) What sort of level of bike you'd be happy with groupset / quality wise?
I'll lay out the options as I see them to get you started. Perhaps if you can also guide us with the following data I and others will be able to give more detailed advice.
1) How is / are your ride single bikes specced?
2) What size are the bikes?
3) What are your position measurements, in particular, how long is stoker's top tube and stem?
4) How do you like going downhill fast? How would you rate your skills?
The Co-Motion and Santana or custom tandems will do all that you need. Price starts at $3200 for a basic Primera. Basically as you pay more you get upgrades in the parts and frame material. Advantage is that the Co-Mos handle very well, are all sufficiently rigid and have a long-enough (28.5") stoker compartment so that most stokers will be happy. Parts even on the low end models are good enough that a strong team will smoke a slower team and can be upgraded, though this will cost more than buying the bike you want at the start. Disadvantage is that the lower end models are a bit basic spec compared to the carbon Dura Ace bike you may be used to, and are also heavier than some of the more expensive models. Other disadvantage is that holding back on the options requires some restraint. As you get towards the top of the range you get more racing-oriented bikes that are more for credit card touring than loaded touring.
I'd say in general that Santanas seem to be a bit more touring oriented by design than the Co-Mos, which means slower handling and some quirks; in particular they use a proprietary rear wheel spacing of 160mm, which gives a longer bottom brackets and only allows Santana spec rear wheels to be used. The longer bottom bracket isn't a problem for most, but if you're riding a Campag record 145mm Q-factor crankset on a single bike, you may feel a bit like a cowboy on one of these. Price is similar to the Co-Mos
The Trek T1000 / 2000 is a nice 'racy but not race only tandem for an reasonable price' at $2200 / $3300. Comparing them with an aluminium frame and similarly equipped Co-Mo it probably has slightly slower handling and a shorter stoker compartment and is a bit more mass produced rather than semi-custom. Not sure about how the weight compares. Difference between the bikes is all in the components and fork, basically the better parts cost a lot more than $1100 to buy separately, even at ebay bargain prices so are a bargain if you ever plan to upgrade. Note that the racy-looking wheels on the T2000 are actually bombproof as well as being pretty light. Top marks in Tandemgeek's reliability survey for pre-built wheels. Stoker compartment length is 28" - 28.3" depending on size.
Final mid range option is the Cannondale tandem at $2600. In a nutshell it's a Trek T2000 with only minor differences in componentry which are 1) it uses disc brakes and aluminium fork 2) frame is stiffer and heavier and a bit slower steering 3) no racy wheels 4) flat rather than drop stoker bars. Fit is a little bit different, with either a 28 or 29" stoker seat tube depending on size. Main difference is the brakes though.
Below the Trek T1000 there are only a few options that are long ride / raceworth, and these tend to be more solid, with lower grade components. One to look at is the KHS Milano at $1600, which is on the face of it similar to the Trek, but with 5% lower performance parts for almost 30% less money. Biggest compromise is the stoker compartment length of 27" and the fact (based on geometry chart) that it only comes in one size which is definitely too small for both you and the stoker.
Broadly, get the bike that can fit your stoker and you without too much compromise. Anything else will frustrate you. For example my wife is 5'6" and rides a 50cm men's frame with an 8cm stem. Her position on a M Trek is 2cm shorter than her road bike, and could perhaps be stretched a bit more, but would end up with her face rather close to my rear. You and your stoker then need to weigh up $ saving versus possible need to change her position. After that you can choose between more or less racy, and go from budget to expensive. In my view the Cannondale RT1000 is probably the best all-round bike in this price range, while the Trek T2000 is best if you want a more racing oriented bike.
Final note: When you compare geometry charts for the tandem, remember that the front b/b rotates to tightent the timing chain, meaning the captain's saddle may
need to go further forward on the seat post than on a similar-measuring single bike. Secondly don't just compare head tube length to check that the captain's handle bar height can be accommodated since tandems generally have taller forks to take bigger tyres than road racing single bikes. Don't get caught out - take a test ride or several.
I bought a Trek T2000 as it seemed excellent value for money and was discounted in a local shop. Other locally available alternatives around that price were a bit touring oriented for us and we're never going loaded touring. Ours (2005 version) has room and braze-ons for rack, mudguards, drum brake etc, but the newer ones seem (based on close up photo of frame) not do all this as the fork and probably also the rear stays only to have mudguard holes but not bolt-on rack fittings. However there are numerous solutions to accomodate this - from P-clips on the frame, other custom frame fittings to front fork mounted rack fittings. I'm very happy with it, and am just in the process of changing to a carbon fork and adding a rear disc brake as I didn't like the cantilevers it came with.