Although not tandem themed, we just ordered matching men's & women's jerseys from Nashbar. They have some cool looking Italian retro patterns from Marilena for $20 on clearance. I've attached the one we picked, although I should add that there's a slight difference in materials, with the woman's having some lycra so it's a bit shiny.

We're wearing them on a ride for for the first time tonight. Oh, and we also had our son create a compter graphic and made t-shirt iron-ons with special printer paper...that's another way to get matching outfits.