If you do not choose a specific platform when setting up your test (iOS, Android, etc), you will get a mix of testers across all platforms.
If you want to run tests on multiple platforms of your choice, you have to run separate tests for each one, with different links and instructions on how to get your app.
Once your first test is live, you can duplicate the test from the "Manage Your Tests" main page by clicking on the icon on the right of the test. You can then make the necessary changes to add a different device platform and instructions.
You can run multiple tests per month with your subscription, so there is no additional cost for splitting tests by platform.
If you would like to target multiple platforms at once or segment testers further, you can contact us to get a quote for our Managed Plan.