Contract and Equipment question

Hello,

I'm currently NOT with ATT but am planning to get the new iPhone.
I have a question on ATT's policy on Equipment and Contracts.

Since the iPhone is not discounted I was thinking of getting a pda phone (maybe that Samsung one) and get it for a discount under contract.

Then purchase the iPhone at full price (no choice really) and activate that afterwards. This way, I'll have a backup phone or I can give the Samsung to my mom.

Will I be able to do this and still be under the 2 yr contract? Or will they add it together and I'll be stuck for 4 years?

Thank you