score:7
One post is the exit control for the country you're leaving. The other is the entry control for the country you're entering.
The country you're entering is concerned that you have proper insurance and registration documents for your vehicle. They will also be exercising customs control over goods you're bringing into the country. In addition, they will of course be checking your immigration documents.
The country you're leaving is less concerned with your vehicle or other goods. They are only going to check your immigration documents to ensure that you haven't overstayed.
Upvote:5
To add to @phoog answer, it's also possible that different agencies are in charge of customs (issues around import and export of goods) and immigration.
To give you two (somewhat outdated) examples, if you use the French sector of the train station in Geneva, you will walk through four different areas: French police and customs and Swiss border guards and customs. That's not the way it works anymore (among other reasons because Switzerland is now part of the Schengen area) but you can still see all four offices/booths along the corridor. Entering Switzerland from the other side, on the German border, I once underwent three different checks on a train: Local police, federal border guards and customs.