I think you'll find that an ATPL is only required if the flight is a Public Transport flight.
If the flight is purely private then 2 x suitably rated PPLs could undertake the flight.
Schedule 8 ANO says the privileges of a PPL are to act as PIC of ANY aeroplane included in Section XII of the PPL.
The chances of this happening are pretty remote but the point I'm making is that it is possible, as the regulations currently stand.
IIRC, in the old days (pulls up a sandbag and starts the lamp swinging) there were weight limits imposed on licences, 12,500lb for a PPL, but these were done away under JARs.
But I believe Mike Cross is right about the cost sharing - oh, bu**er, I'll just have to pay for the whole thing then!!
