To me, the most logical is Pamela hitchhiked East from Colorado, and somewhere on the East Coast she met up with James.
Maybe he had a car that he picked up sometime after he left. Most guys during that time knew how to work on cars and he might have picked up a junker and got it on the road. He could have either picked up Pam and that's how they met or by coincidence they just both were hitchhiking at the same location and decided to join up.
If he had a car perhaps they picked up someone local who killed them and stole all their stuff and got rid of the car or they were picked up by a local and killed and then all their stuff was stolen.
So many hitchhikers were killed during that time, that it wasn't all that unusual. The only unusual thing about this case is how long it took to identify the two victims which was, I believe due to tunnelvision, where it was just assumed that they were a couple and not two separate missing people who might have met up.
Wonder if they still have any of the clothes for testing of possible touch DNA?