Originally Posted by
littlefish
Interesting, useful to know. BAEC can do this when booking the second (return) leg for avios bookings.
I would therefore imagine its a VFC limitation.
I think the difference is between booking the outbound and then calling up and converting it into a return/open jaw when you book the second flight, vs booking two separate one way tickets and then asking to merge them into one ticket.
As far as I know, if you book two separate flights with BA they also don't allow you to then combine then after the fact, although as you say if you book the outbound you can convert it into a return when you book the second leg (as do VS).