Thanks, Nanacake. The only modification was the creation and placement of the other sim, which did not require any hack or cheat code. I created the sim (no custom content) and added them to a household. When the sim from Sunset Valley went to visit, the sim I created in France appeared as the food vendor in the café.
I'm thinking the relationship has messed up being able to travel back and forth - a seperate neighborhood is able to travel back and forth with no problem.