Book the outbound flights, one-way, with the companion voucher(s) as soon as they become available. Once the return flights open, call up and ask to add the return leg to the existing booking. This isn't normally possible, but there's an exception in place for when the return flights weren't open for bookings at the time you booked the outbound.

I'm assuming here that you're going for the reward ticket inventory that's guaranteed on each flight at 355 days ahead. If not, then you're in much murkier waters.