I have used
Undercover Tourist several times and have never had a problem. Like shocker said above, you cannot open the clear plastic sleeve that your tickets come in if you might want to return them and they cannot be returned after they are linked to MDE. I also kept my tickets in a safe place in the plastic sleeve until the day before Fastpass+ ressies could be made. I had no problems linking the tickets.
When you do link the tickets in MDE, you assign each ticket to an individual. So just make sure you assign the 2 day tickets to the right people and the 7 day tickets to the right people, and you will be all set.