There is a way to cheat, if you don't care much about details. Take a medium size pneumatic staple gun or screw, and screw/staple the lower piece (as close to the joint as possible) through the S lock of the top piece into the stud behind it. Otherwise, you will have to take off a bunch of metal below it to access the S lock by itself. The trailers are sided at the factory from the top down. To fix it, work from the top up.
If it is a lower piece of metal, sometimes the staples going through the bottom up into the floor come out, and that piece of metal sags down a bit. In that case, just staple it at the bottom.
Can you post a picture of the seperation up close, and from a distance so the surroundings can be seen?