How to use GarageBand on iPhone and iPad with another person at the same time?

I seen it on the iPad advert so I know it can happen. I have it on both my iphone and iPad and me and my cousin want to play it like on the advert. All help is appreciated

When you click on the instrument, near the settings, there's a button that has a Wi-Fi signal coming out of a note and when clicked on shows up a notification for Bluetooth. Click OK and it will search for another device. Do the same on the second device and it should connect when either one of you clicks on the device name.