The 2 day park to park is $130 with tax included. From Universal a 1 day park to park ticket is $136 + $8.84 in tax. So buying the 2 day ticket and throwing the second day away is almost $15 cheaper than a 1 day ticket. Plus, you have a second day if you can pop in even for a couple of hours.
The biggest down side to those tickets is they are date sensitive and the can't be upgraded.