Our opinion:
Calfee or Rue for carbon fiber. Have ridden Calfees and seen/hefted Ying Yang Rue.
Co-Motion for alu or steel. Have ridden all sorts of Co-Mos. Seen/hefted the Macchiatto.
Magnesium: Paketa; have seen/hefted, but not ridden.
Ti: have ridden 2 full Ti tandems: Serotta and Santana. Nice but not overly impressed
As for Santana . . . they are good tandems but not racing stuff; they've never won either the Burley or Co-Motion Classic tandem race in Oregon; it was 'rumored' that Mr. Bill offered $$ for a 'tana win.
Still waiting for that.
Brakes: calipers; your choice of brand$/pads.
Weels, durable + light: Topolino.
Stoker bars: her choice! Try drops; if they don't work out, cut 'em down/flip 'em over and you have cowhorns.
Pedal on TWOgether!
Rudy and Kay/zonatandem