If you have created a Mobile only campaign dedicated to a specific OS (either iOS or Android), we have set up the option to duplicate this campaign and focus the other OS you had previously excluded.
The best part? It's crazy simple!
First, open the report for your mobile OS specific campaign. Once you're in there, in the top left you'll see the option to duplicate the campaign, and to duplicate it to the other OS, like this:
Clicking on "Duplicate for Android" (or for iOS if your original campaign was targeting Android devices) will open the campaign creation flow on the Summary step, with an audience targeting only Android (or iOS) users.
You can of course change information if you need to in previous steps of the campaign, as you would in any campaign duplication!