HI Daniel. I do not believe you would have to purchase the app twice. I actually had my LeapPad wiped clean due to an update and the downloads where right there in my download center where it shows just which apps I had previously purchased. I just clicked the + sign and downloaded the apps again. I called Leapfrog and they walked me through it. Just to be certain I'd call if I were you. The customer service reps are very helpful.
I did this today. You can add to 3 devices. The catch is that the download has to be from the same parent account. I downloded it onto one leappad then all I had to do was plug in the other device and it synced the new apps to the other device. It is quite simple to do. If you go to leapfrogs site under the FAQ it is the first question on there and it gives step by step instructions. Good luck and enjoy the peace and having your smart device back.
For only two, no you won't. Like an ipod, you can put several devices on the same account, and the apps that you buy will be able to be downloaded on all of the LeapPad 2's on your account. You can only add so many LeapPads. I think it caps at 7.