I went to the ATT store and asked this question. I got the dear in the headlights look. He then went behind the wall and asked a knowledgeable person. He said to restore the ipad 2 from the ipad one backup and then take the sim card from ipad 1 and put it in ipad 2.
Really, I still didn't feel great about these directions, but it seems to have worked fine. I still have the unlimited plan and in fact it renewed itself about 3 days after I did the above procedure.
I have heard that you can call ATT and they will switch it over to a new sim also, but that seemed like a bit much.