If you want to run your playtest on 2 platforms you'll need to start two orders. Specify the number of players you want per platform in the orders. For example, if you want 6 players in total, place two orders for 3 players each – one for iOS, one for Android
All you need to do is:
Make an order on the platform, being sure to give us: the build, the specified target device.
Go ahead and place the order, then return to the my playtests tab and find the order.
It’s as easy as that!