You do not make (nor can you make) a park reservation for a second park
You MUST enter the park for which you have the park reservation that day, or you will not be allowed into the second park.
The park reservation calendar does not indicate park capacity for the purposes of hopping. To date, there have been zero reports of anyone being denied entry at the second park. This includes every day in the past month where the reservation calendar showed no availability. Cast members at the bus stops at the parks can tell you if a park is at capacity, or you can call (407) 560-5000 to get that info. But again, so far, no park has disallowed hopping.
You can leave and return as much as you want to the park for which you have a park reservation.
Enjoy
Steve