Correct - room key and park tickets are separate media. But.... your room key and your Express Pass are the same thing! Confusing, I know.
Here are the key things you need to know (pun intended)
- You'll need to scan the barcode of your park ticket to get in the parks or to board Hogwarts Express
- Pretty much everywhere else, you're going to be scanning your room key, including
- Getting in your room
- If you use Early Park Admission, once you are in the park, there's a checkpoint where they will do a quick visual check of your room key (no scanning happens)
- You can add a credit card to your room account and then use that room key to scan and pay
- Finally, to use Express Pass, you will have to show/scan your room key. To enter the EP line, a TM will visually check your room key. Part way through the queue, another TM will scan it.
- To get a locker, you need to scan a barcode - either your room key or your park ticket will work for this, but I recommend that you (and all of the signage will tell you to) use your room key and stick with that. One time, I reserved the locker with my room key, then scanned my EP to 'retrieve' my stuff. Imagine my moment of panic when the second locker opened and there was nothing inside! Showing my age, it was like Geraldo Rivera opening Al Capone's vault.
I now find it easier to just use my room key every time - it's easier to remember that the room key opens 'doors'... to the room and to the locker. 
Lanyards with clear pouches are pretty common at Universal for the above reasons. Put your room key barcode facing out on one side, and your park pass barcode facing out on the other. Easier than having to dig things out of your pocket over and over.
