Originally Posted by
Ldnn1
That's not correct. You can call BA and they will do this all on one booking for you, assuming availability of course.
(I read the OP as wanting to pay for both flights with avios, but combining F+Y, which cannot be done online but can over the phone. If OP wants to pay for the Y flight in cash, then no that wouldn't be possible.)
As others have jumped in to say, you can't combine an Avios booking with a cash booking, especially on another airline.