There are often as many names for those two cuts as there are for porn actresses.
I have heard the underneath cut called the "hanger loin" and even the butcher's loin, because the butcher often used to appropriate it. Some people will call it the tenderloin and the big one the backstrap.
When my wife was writing her last big game cookbook (which partly involves butchering at home) I came up will the names "overstrap" and "underloin," which seems to cut down on the confusion.
