Another mum at the playgroup is expecting twins and is looking for a double buggy travel system. I remember seeing one in use last year, but can't remember the make. It was a tandem-style, but not as long or as bulky as usual. I think it was the sort where the buggy frame can carry different attachments, and had two carseats attached when I saw it, rather than pushchair seats (I could be mistaken about that though). Both front and back positions could face backwards or forwards independantly.
Does this ring any bells?