Don't see why you would care whether you use one relationship or two to get the job done. What you describe is the scenario for a "basic conditional value list" and should require just a single relationship so I don't see where a second relationship would be needed for that. I personally would use two as I would be linking records by ID and would use one relationship to match by customer name in order to look up the ID to link the records by contact ID, as that makes for a more user friendly approach and it would not bother me at all to use two relationships to two diferent occurrences of the contacts table to make that work.
For a working example of a "basic conditional value list", click the button by that name in:
For a working example of how to set up a name based, auto-complete drop down list where the user selects a name but the records still end up linked by ID, see: