If you didn’t set up it to cancel something, it will only fill for both nights even though you have one already.
What it means is that if that one night becomes available and your waitlist is for the original 2 they will skip right over you so in essence it doesn’t matter where you are since they don’t hold the night.
Being on new waitlist for just one night would actually jump you ahead of your current one if that night comes in.
That is what I always do. Now you could call or chat with MS to see if they can adjust it without changing your place but typically it does.
Going to add to go back and check for the room if you cancel the waitlist because if it did fill and was waiting to be processed...nothing shows that..you may find the night there.
That has happened to me a few times when o took my second choice and canceled the waitlist for my first one.