Demo, demo, demo. Find a shop that supports and rents tandems and ride around on flat terrain first.
Starting from standstill is different and requires planning and practice. Braking is different. Downhills are way different, way faster. Climbing is harder.
The stoker can have serious comfort issues that aren't remotely apparent to the captain whilst riding over the same terrain.
After your purchase, it helps to belong to a tandem club because route planning is often different than for singles.
DaVincis are nice, but you -knew- the gear geek in me would say that just based on the gearing options.