Yes, it will need to be available to book as one reservation. When they go to book the additional flights it will require a new ticket be issued. This is an issue since if you book a one-way from Europe you get hit with the Europe origination surcharge. However most want to book the outbound to Europe as soon as possible. Leaves you with the choice of
1)booking the outbound while you know it's available, and waiting to book the return, but paying the origination fee, or
2)waiting and booking it all at once, saving the origination fee, but running the risk inventory on the outbound being not available.