Welcome to planDisney! Thanks for bringing your planning question! Nathan, what you are looking to do is affectionately known, in Disney circles, as Park Hopping! And, I have good news! Guests are once again permitted to hop between the Disney Parks!
In order to enjoy more than one Disney Park in a day, Guests must have a
Park Hopper ticket. This ticket is available for purchase when
placing the order for your park tickets. There are a few important details that you should know about park hopping. Guests must still make their park reservation for the first park that they wish to visit. This is done using the Disney Park Pass Reservation System. However, Guests need not make a reservation for the second park that they want to "hop" to for the rest of the day. Currently, park hopping is permitted to begin at 2:00PM. You can remain in the park that you "hopped" to until closing time for that park!
I think park hopping is a great option for adults and parents with older children. Park hopping can get a little more complicated with younger children or older-adults as it feels like a very long day. Now that we have teens/tweens, we like the flexibility that park hopping adds to our visits. The example that you provided is correct! You can begin your day at Magic Kingdom Park (or any other park) and then hop over to EPCOT (or any other park)! So, you can begin your day atop Space Mountain and end your day on Expedition Everest! Not a bad way to spend a day!
Hakuna Matata! Jonathan