Cars, Bikes & Driving
I've never heard about a tandem club, but the FSCL might help (Luxembourg Cyling Federation). FSCL registered clubs: http://www.fscl.lu/clubs/
Otherwise, a great way of meeting cyclists in an informal way is to participate to Alles op Velo, a yearly event organised the weekend of July 15th. There are definitely tandems participating. More info will be published in Lux-Live end of June or early July.