This is a little bit complicated and will require some work on your end.

First of all, although you cannot get a two month account on Orange, you can get a prepaid number on their Mobicarte service. Orange does sell the iPhone (in fact, I was thinking of changing my Orange phone to iPhone the next time I am there). They even sell the iphone unlocked pursuant to French law.

Here's where it gets tricky. I do not know if an Orange store ("boutique") is equipped or willing to unlock your (I assume foreign) iPhone to use their service.

But, Paris is filled with businesses that specialize in unlocking cell phones. Look for signs that read "Portables Débloqué" or "Portables Déverrouillé." Or perhaps someone at Orange can point you to the right place. If not, the geeks in the cell phone section of FNAC on the Champs Elysées might help if they can get to practice English with you.

Bon courage! (Roughly "good luck!")