unless they are more than 6 inches apart. you dont really need to put somthing in the middle. but an idea would be something like liquid nail, just put a line of that down the center, but dont let it sit proud, or your rail wont slide well.
if ur mega worried about it build the end of the first piece of pve a little bit higher lighid nait wont work maybe find a pvc coupling and cut it in half and use that piece under where your ski doesnt slide to join the two? my friend did that worked great