Unable to find in App Store, able to redownload using iCloud?

I obtained an app from the App Store that I can't find now or was it simply taken down, but I still have the app on my iPod. I'd like to install it on my other iPod.

I don't know that if I backup my iPod (iCloud backup) it'll still be there.

Can I install the app from my backup to my other iPod and still be sure to keep it on my original device?

Connect each phone to iTunes over USB - choose to transfer all purchases to the computer. When all the apps are on iTunes, you can install them on devices that them even when the store online is "out of stock"

In iTunes, click on the Apps icon to see a grid of your downloaded apps.

Plug your second iPod into your Mac/PC.

Drag the app to the new device. (It should show automatically in the sidebar as you start to drag.) [Assuming you are using the latest version of iTunes. It should work on previous versions as well.]