Sync them both to the same iTunes with the same apple ID.
However be aware, while most iPhone apps work on the iPad, many iPad apps are optimized for the larger screen and dont' scale down. If your app is labeled HD then it may not work on the iPhone. However, in the App store if it had a + by the name then it's a universal app that works on either device.
ETA: then I read it's Snapseed...yeah it's universal. I have it on both my Touch and my iPad.
You can also try just connecting your iPhone to the App store and if you've got 'automatic downloads' enabled the app will be waiting for download to your phone.