I've found this thread to be very interesting! I guess the safest way to back-to-back ticket is to use two airlines that don't have any relationship with each other. One example would be US Airways and Continental. You would place the miles in the respective FF accounts and neither airline would know. The only bad part is that it would be harder to get elite status because you wouldn't be able to combine the miles.

However, does anyone know if airlines share information on passengers? I would guess this to be unlikely for non-partners. But what about NW and CO (as an example)? If you back-to-back ticket on those two and place the miles in two separate accounts, could they catch you?