My preference would be to keep it as a one-way process and issue the tickets as one-way tickets.

Otherwise, you run into the problem of one direction clearing at one time and the other one either not clearing or clearing at a much later date, which is difficult to handle from a ticketing perspective. The key to making this work is that once the seats are allocated, the top name on the list has to either commit to them or not; to commit to them contingent on the other leg of the roundtrip clearing would hold too many seats in limbo (seats that could go to others on the list), so in my mind once one of the one-way components becomes available, it needs to be claimed and ticketed immediately.

If the other one-way leg never clears, then the customer has the choice of either buying a one-way ticket, or canceling the leg that did clear and redepositing the miles (with the fee that goes with that redeposit).