Using Ipad to control a third deck

My set up is :

2  x   XDJ 1000mk2,   1 x djm900nx2,   1 x ddj-xp1

Its all linked up through ethernet. My question is hopefully simple to answer :

Is there an app for ipad that will allow me to control decks 3 and 4 on the rekordbox software ? Im happy using the ddj controller to do most things - I just need the ability to nudge the waveform forward or back if the beat is out of time.

Ideally it would be ipad, but if not, is there a cheap usb piece of hardware that could do the same job ?

Thanks in advance