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Since you have devices with separate Operating Systems, you will have to purchase the Application at least twice; once for the Android set and once for the iPad.
So long as you use the same Google Play Store ID for the Android devices each time, this will allow you to download this without extra purchases an infinite amount of times. The SlingPlayer for Android Phones will also work without issue on Android Tablets.
Same goes for the iPad application, in a way. This can be downloaded on up to 10 Devices without extra purchases so long as you use the same Apple ID for the purchase as you do for the downloads, however, the iPad application will only work on other iPad Devices.
I hope this helps!!!
Sling Media Moderation Team