Goeden dag Marten!

While not per se a handlebar bag, what seems to have become popular in the US is what is called a Bento Box.
Attaches with 4 velcro straps to pilot top tube and pilot stem; possibly could be used in stoker position also. Ideal for carrying digital camera, snacks, etc. Have not tried one, but seems to be a bit of a buzz about them on T@H
www.tandemseast.com apparently has them for sale. Small Bento @ $15, large @ $18.
Might be worth checking out.

Pedal on TWOgether!
Rudy and Kay/zonatandem