While you can't directly use an iPhone to control video recording on an iPad. I found an app that uses Bluetooth to allow one iOS device to control the record button on another iOS device. https://itunes.apple.com/us/app/rego-remote-video-recorder/id603028659?mt=8Is there a way to use the iphone as a sort of remote control for the ipad where you can use the iphone to remotely tell the ipad to start/stop recording?
Also, is it possible to see the live recording of the ipad on the iphone?