Remember when we used to play games with people who were actually in the same room? Rookie Play Store developer Seabaa does. They've created DUAL!, an Android game that positively demands you play it with friends. DUAL is basically a top-down space shooter in the style of Galaga, but the structure has been modified to allow two people to play across two Wi-Fi connected devices, either competitively or cooperatively.
The primary game mode pits two players against each other. Once you're connected (and you figure out which way the screens are supposed to be oriented), tilt to move your pixelated ship around the field, tap to fire short shots, or tap and release to charge a larger shot. You'll have to do all of this while avoiding the incoming fire from your opponent on the other phone's screen. Thanks to the Wi-Fi connection the action is fast and fluid - the shots jump across the screens smoothly. If a larger phone or tablet is connected to a smaller one, a portion of its screen will be blocked off, giving both players the same amount of visual and virtual space.
Things get more interesting in the Defend mode, which costs a reasonable $1.99 to unlock. In this mode the dual-screen playing field is roughly the same, but you're both shooting at enemies as they appear on either screen. A swipe action will let you reverse your ship's direction, and red or green enemies can only be destroyed by the corresponding player.
Though DUAL! is available on Android and iOS, it doesn't work across the different operating systems at the moment. It also requires both devices to be on the same Wi-Fi network, though Bluetooth support is coming in an update.