Sorry, you'll find that the Amex 2-4-1 voucher (at least the UK version) is valid for return (aka "round trip" for NA cousins) bookings only.
You WILL have to specify a return flight, but can use that as just a placemarker, and change it for the date you actually want if availability comes up later.
If you end up on your AA single redemption (and "no show" for the BA return flight) then arguably you will be no worse off than if you had booked two single redemption tickets on BA without the voucher (2 x 40,000 = 2 x 80,000/2) except that you will have 'spent' the voucher for no benefit.