Someone (possibly mersea trailers in Colchester?) does multi deck trailers for dinghies. If you could persuade them to do a sib friendly launch trolley then job done, and for the moment put the oppie on the car roof. When the kids inevitably grow up you can then park the laser / 420 / whatever they grow into on the upper level of the trailer.
Google "multi dinghy trailer" and you'll get some suppliers.
Other option is a "ladder" trailer which is essentially a camping trailer (but a decent one!) with a 2 or three rung "ladder". Usually used for carrying multiple lasers / toppers / Oppies, but if your Sib is one that the engine comes off easily, there's nowt to stop you using one of the levels for it.
I've seen many adaptions of Rib trailers to top a dinghy over a rib, so someone out there will do one to suit a bunk trailer.