Dave,
I've only had toppers with one spring but several regular awnings with two springs. The installation instruction mention there can be two springs depending on length.
Depending on the length of the slide, on one or both of the end caps there is a hole that will allow you to lock the spring. They come with a cotter pin inserted through the hole and into the spring shaft, from the factory, which is remove once the topper is installed.
Put the slide out and with your hand unroll further enough material to allow you to wrap the awning material one more turn on the tube, then continue turning until you can slide something about 1/8" diameter(cotter pin, nail, piece of stiff wire) back into the hole to lock the spring(s). Make sure the pin is actually holding the spring back. Some times they are a bit difficult to find the inner locking hole.
Remove the screw to release the awning tube from the end caps and role the material on the tube one more turn. Re-install the tube in the end caps. Hold the tube and remove the locking pin and allow the material to slowly roll up.
As with regular awnings, allowing the spring to unroll uncontrolled may damage the spring, so hang onto it. If you have to let go, unroll it first, you can always roll it back up, just takes more turns. Manufacturer usually specifies how many turns tension should be on the spring in their spring replacement instructions if they exist. I have put an extra turn on a couple to tighten them up and haven't had any issues.
Joe and Evelyn