Hi!
That is a common question, but unfortunately the answer is "no". With a base Magic Your Way ticket, you can only enter one theme park on each day of your visit. If you want to visit a second theme park on the same day, you have to have the Park Hopper option. In other words, in your example, you can't use "Day 6 or 7" of your 7 Day ticket on two of the other days. Disney keeps tracks of "days" and not admissions. With a 7 Day ticket, you have 7 days you can enter a theme park, not 7 admissions that can be used in less than 7 days.
You can get more information on tickets
here. By the way, you don't have to buy the Park Hopper option up front. You can wait and see if you actually need it. If you visit a second theme park on the same day, then you can simply buy the option then.
Enjoy planning your vacation and write back if you have any more questions!