I have several genymotion emulators for different API levels. The problem is this:
After this, the app only runs on the first emulator and I can't figure out how do I show the dialog (select target) again to run the app on both emulators.
You can select a single or multiple devices directly from the toolbar:
The answer below is for versions older than Android Studio 3.5 - where target devices hadn't been split from run configurations:
Click Run → Edit Configurations...
Then uncheck Use same device for future launches
As mentioned by @B-GangsteR, you can also launch your app on multiple devices at the same time. To do this, select targets using ctrl key.