You’re in luck: iPod touch apps and iPhone apps are synced to all the compatible devices you own.
Just as a song or album you buy an the iTunes Store can be synced to all your compatible devices, the same is true of App Store items.
Simply download the app the you want – either from your desktop or the over-the-air App Store. When you sync your devices, the new app will be automatically added to it unless you’ve changed your sync settings to only certain apps.
If you download an app over the air onto your iPhone or iPod touch, when you next sync the device, the app will be added to your computer and then to your other devices next time they’re synced.