I believe I found the problem. I was able to use Endpoint-1 to pull only 1 account. I was successfully able to refer to the UserID account attribute with $response.UserId$ variable in Endpoint-2. It contained the employee ID and successfully correlated.
I did this by changing the Root Path to $.Entity.UsrIdLst[0].Usr. The object in slot 0 was pulled only.
It appears that the problem is Endpoint-2 does NOT iterate through each user from EndPoint-1. I get an Error 500. It appears Endpoint-2 is trying to send ALL the users in one call to the endpoint.
Can you all confirm this is the expected behavior? Multiple endpoints do not iterate through the array returned from parent endpoints? It only does one call with the whole response array?