We own/ride an ariZonaTandems custom carbon fiber 2-seater; currently 8,000+ miles on the odometer.
Have also ridden a Calfee Tetra Tetra for 100 miles a few years ago (we were doing an article for a magazine).
Both tandems are impressive: beautiful, light, easy to handle and great for climbs.
We no longer 'race' (we are septegenarians) but do appreciate finely crafted/handling machines.
Have ridden 'bout 30 brands of tandem in the past 30 years; have owned five tandems (a Follis, and custom Assenmacher, custom Colian and custom Co-Motion); have pedaled 200,000+ miles as a tandem duo.
My stoker, Kay, sort of got tired test riding different brands of tandems and is not easily impressed. We've test ridden well-known, and lesser known, brands like Gottfried, Berry, Gilmour, Dawes, Serotta Ti, C'dale, etc. Having said that, both the Calfee and ariZonaTandems impressed her and me, immensely.
Carbon is the 'new wave' material for tandems.
ariZonaTandems standard frameset is $4,700 and that includes c/f fork, extra-lite eccentric and set up for caliper or disc brakes, clearcoat or single color.
Options galore (more $) from custom sizing/geometry to c/f adjustable or non-adjustable stoker stem, or integrated seatpost/stem/stoker bars, or c/f stoker
handrests. Choices of forks, paint, etc.
Calfee's prices are a bit higher.
Davis will build just frameset or delivers complete built-up tandem. You decide.
You questioned wheels. On our tandem we use Velocity Aerohead rims (32H front, 36H rear), Chris king hubs, DT Revolution spokes. Ti spokes, of course could save a few more grams but add $. Rolf wheels would be another good option.
'Custom' is all about what you want and what you are willing to pay. Davis even hand-crafted window cut-out c/f lugs, a c/f rear rack and a braze-on (glue-on) for our remote garage door opener, plus a spare spoke hanger under the boobtube. The sky can be the limit (but wallet will dictate)!
Hope this is of some help.