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At least on AA, the 35K one-way Coach award is an AAnytime award, which can be booked anytime, as long as the flights are not completely sold out. Not sure if the 35K UA awards work the same way, but if they do, you could just book the outbound on AA now, using BA Avios. Then you could use AA or UA miles later to book the return. It's even possible -- but not assured -- that SAAver seats will open up on AA or UA for the return in the interim.