Selecting a start and an end for one record


I am trying to create a field/form where users select a start and a finish destination for a trip.

It could be a drop down selection or the user could add the record in if it does not already exist.

There also could be multiple start and finish records for one trip.

For instance,


Start: Point A End: Point B

(Add record if needed)

Start Point B: End: Point A

Any help is much appreciated.