‘It can't be done online’

You can do the booking online, if you use the two separate bookings method, the return with full Avios then you have two reservations, so potentially two lots of cancellation fees etc. I did this for my last open jaws booking ( to the US) but then you have to call anyway to get the Avios refunded.

And as OP appears to be US based, if they want to secure seats as soon as they are released, they will be calling/booking at a more reasonable hour anyway than us UK lot!