Easy as I do this a lot. (A booking with the outbound and return being parts of two separate trips.)
Set your account to place all new bookings sent by email into "Unfiled Items".
What I do it them 'copy' the flight booking data to each of the intended separate itineraries, deleting the unwanted segments as I go. The last one I 'move' rather than copy so it's no linger 'unfiled'.
e.g. For a simple Return:
- Email your flight booking.
- One acknowledged, go into your trip it account.
- Select Unfiled Items and find your return flight booking.
- "Copy" it and when when edit comes up, delete the segment for the return journey.
- Complete by moving to a new or existing outbound booking as appropriate.
- Go back to "Unfiled Items" and find that booking again.
- "Move" it and this time, delete the outbound journey before moving it to the other new or existing inbound booking.
Good Luck.