Sidecar is designed for the iOS 9 iPad "slide over" and "side-by-side" mode. It gives you a handy and configurable set of MIDI pads, which you can triger with an ordinary iOS QWERTY keyboard. Sidecar works as an AUv3 plug-in too! More info
If you have a Behringer FCB1010, you need FCBFF, your Foot Controller's Best Friend Forever. The app will program the FCB, and can emulate it too. The app supports slide-over and side-by-side, and you can use a standard iOS QWERTY keyboard as well.
Stream low latency audio between a pair of iOS devices, using WiFi -- or connect two apps on the same device, if they don't have full support for IAA and AUv3. The app can run stand-alone, or as an AUv3 plug in, with receive and transmit variants.