the tandem Club have a booklet on carrying tandems by car. It strongly advises against carrying tandems across the back of a vehicle because of the legal complications of the overhang if there's an accident - that advice was inserted on solicitor's advice. It speaks highly of the BTS but the person who wrote the booklet (me) was going off TC member experience generally as I've not used one myself. I used a home-made horizontal-on-top when it was just one tandem, then a home-made vertical-on-top (same concept as Pendle) when it was tandem + adult solo + assorted child solos on top, and now a trailer (bought second hand and adapted at home) which recently took 2 tandems, one adult solo and three child's solos to the Lakes.
http://www.tandem-club.org.uk/nf2002/faqs/tandemsbycar.pdf