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.
I stand corrected.
However, ATPL theory passes are required in any case to get the type rating.
And in practlice the TR is likely to be limited to co-pilot only.
The chances of this happening are pretty remote but the point I'm making is that it is possible, as the regulations currently stand.
Example how the FAA treats this: John Travolta has only a FAA PPL. He has a B-707 type rating, but the limitation "B-707 SIC privileges only" entered on his licence.