The cheapest way to get the 2 day base passes is directly through Universal online. Not only do you save $20 off the gate price (you can get 2 day base passes for $115) they also offer you to upgrade your tickets to PARK TO PARK tickets for $10 instead of the normal $20 you have to pay to upgrade to park to park.
I would suggest you get the park to park because IOA is the better park and once you go through Universal you will want to hop back over to IOA. Or at least we did. Universal sucks compared to IOA. It's still a good park but not near as good as IOA.