TH is right. If both iPads are using the same Apple ID account then you can simply download the app again on the other iPad. Just go to the Purchased tab at the bottom of the App Store, find the app and install it.
If you have two accounts, it is still possible, but trickier. Apple allows sharing of app, media, music between family members, but not between accounts. What you have to do is go to the bottom of the App Store's Feature tab. You'll see the Apple ID that you are logged into the store with. Tap it and select sign out. Tap it again and sign in with the required Apple ID. You can now see the Purchased apps from that account and download them for free.
However: from that point on you are going to have to keep track of what apps were bought on what account. They can only be updated when you are logged onto the account they were bought with. This may be more trouble than just buying a 99 cent app (it is on sale as I type). Or just getting the free version and putting up with the advertisements.
Up to you.