I'm still not sure why this idea still persists to this day. It has been like this for ages. You CANNOT use a multi day ticket to enter more than one park per day without a hopper option. Especially after the 4th/5th days etc, those days become very economical (sometimes like $10 or $20) so no, you cannot use them to 'double up' on your park days. That day is provided as a discount for purchasing the 'package' essentially. As Robo says, the only way you could do that is if you burned two completely unrelated single day tickets, which would be extremely cost ineffective (much better idea to add hopping for the entire 5 days and have the option to come and go as you please.) Adding hopping for a single day isn't the best use either (but its cheaper than two single day tickets.) The advantage is that once you add a hopper, there are unlimited hops you can make, so even if you ended up missing a show you wanted to see, you can hop back on Day 5 and only catch the highlights if you wanted.
Base is base, people. One park, one day, the end. (I know MANY of you on here know this. I'm still amazed some don't, and this misinformation keeps coming up.)
OP, of course, you can always upgrade to a hopper, but if you wait until the end of the park days, you lose some of the value of having had it. Say you plan a day at DAK, it closes at 5 or 6, if you had the hopper option, you could still potentially spend till 11pm or 12am at MK and make a totally long awesome day of it, if you wanted, and if you had a hopper. Otherwise, once your first park closes, with a base ticket, you are done for that day.