Yes you can. The computers simply deduct a "day" every time you enter a different park. They don't care when you use it. So if you have a three day pass, you can visit 3 parks in one day (but then nothing for the remainder of your stay).
We were there last March. We were going to be at the parks for 6 days. On only one of those days did our plans call for going to two parks in a day. I didn't want to pay the extra cost to upgrade all our passes to parkhoppers ($40/each $160 total). I just bought a 7 day pass (Extra $2/each $8 total). We went to MK in the morning and then to MGM that night.
Saved me more than $150 plus tax.
Cheers.