My friend Megs recently purchased a corset and ran into a roadblock when the lacing came out... as it turned out to be two pieces of lacing and she wasn't sure how to get it back together. I came to her aid and took photos along the way.
First I unlaced everything completely.
Then I separated both cords and threaded one through the top two holes and the other through the bottom two holes.
I then proceeded to lace both lacings up/down until they met in the middle.
We then put the corset on Megs, I tightened it up, and we drew all four ends together to tie in a bow. The advantage to the double laces is that you can have the top of the corset be a different tightness than the bottom, which can be a real asset if you're having trouble breathing/moving.