I've been burned once, booking BE tickets for my brother. I booked it as a round trip, he needed to leave one day later due to work commitments. Since I booked the entire ticket as one round trip, I had to forfeit the entire ticket. Couldn't split the ticket after the fact either.

If there is even a remote possibility that you will need to change one or the other, and trip in vain isn't a tremendous concern, then book two one-ways.