Also, note that you can't do a free companion ticket *after* booking your own thicket. They need to be booked at the same time, using a companion cert from your DL FF account. However, you could cancel your ticket (for free if it's not Basic Economy) and then book using the free companion ticket cert, but your employer might not be happy about this as you technically wouldn't be flying on the ticket the employer purchased. Your employer might also require refundable tickets or other conditions that might not be compatible with use of the free companion ticket cert, which requires certain fare classes.

OTOH, if you mean to simply pay and take a companion with you on the trip, you can link but not combine itineraries (again, without cancelling your ticket and booking both tickets together on the same PNR in exactly the same fare class, etc.).