My best advice is to drive yourself between the two parks. You only have to pay once per day for parking, regardless of how many parks you visit. Additionally, you don't even have to pay to park at Blizzard Beach, so if you decide to make a whole day of it and go nowhere else, you'll have saved yourself that fee.
It will only take about 10-15 minutes to get from the Blizzard Beach parking lot to the Magic Kingdom parking lot if you take your own car. If you take Disney busses, you might be looking at a much longer trip. You'll be on their schedule that way, which is one of the reasons I always recommend renting a car to anyone going. For me, any time spent getting between parks is wasted time...time that could have been better spent on an attraction. Have fun!
