Clearpass already has the logic built into the captive portal to resize the page based on the device. You can have a custom skin built so you dont have to do any programing to make it work.
What is the reason why you would need two CP pages?
In most cases you can go off the DHCP profile and assign a page based on the device type and send back a different role for each type. The problem with that is that it can be a race condition where the profile might not happen until the device is profiled.
I haven't tested with IAP but you can also use the Aruba VSA and use that as your condition for what type of page to show.
Again that is a lot of rules that you will have to make or couple big rules.
I'm sure there are a few other ways to also accomplish this but those are the few off the top of my head.