Controller Test Jig
This Control test jig was used to test the controller ports in a Nintendo system to ensure that they were working properly. Although these tests could have easily been accomplished by just using two different controllers, Nintendo offered this item anyways. One possible explanation for this could be that it was easier to manage and keep track of than two generic controllers. Also, you were less likely to accidentally use a defective controller when using this to test. However, I haven't been able to find any information on this test jig in my wanderings, so I am unable to verify any of this for certain. It basically consists of the front half of two standard NES controllers and a large black box. The top controller goes to port #1 while the bottom connects to port #2. It pretty much works like you think it would by looking at it.