1) Don't put your FF # in the records. That is one of the primary mechanisms the airlines use for duplicate bookings.

2) Book the two reservations through different booking engines.

3) Modify your name slightly. Use a middle initial on one booking and not in another. Use a suffix such as Jr, PhD, JD etc on one and not the other.

There are many cases were a parent and child with the same name are both flying together. Putting in these small differnces should help, as the airlines will not want to cancel and father and son, e.g.