Hey Howdy Hey Cassidy and welcome to planDisney!
It sounds like you have quite the plan for your upcoming vacation to the Walt Disney World Resort. I couldn't agree more that Magic Kingdom deserves 2 full days to experience all of the fun this park has to offer. With your theme park ticket strategy, you do pose an interesting question and I will be happy to give you more details on how to get your strategy carried out.
In short, yes, you should be able to get your tickets used in the way you would like them to, but this may come with extra attention to your
My Disney Experience app on the day of your first Magic Kingdom day. Stepping back a second, you need to make sure you see all of your tickets in your app. Once you log in, tap the three horizontal lines on the bottom right for more options and then select the tile for "My Tickets & Passes". You should see 2 different products, the single-day ticket should say "1-day ticket with admission to Magic Kingdom" and the other "Walt Disney World 4 Park Magic Ticket"
When you purchase the 1-day ticket, this is date-based so you had to select which day and park to use it. On the day of your visit, after you tap into the front of the park, check on your My Disney Experience app to see that the 1-day ticket was used. Since you had to select that particular day and park, the system should recognize this and default to using it. Technology is great until it isn't so just double-check to make sure this is used accordingly. If not, you can speak with a Cast Member and they can quickly change things around.
I hope this process makes sense and you can get your tickets used as your strategy intends. If you have any further questions please let us know, we are always happy to help!
To Infinity and Beyond,
Nick