Audiobus: Use your music apps together.
What is Audiobus? — Audiobus is an award-winning music app for iPhone and iPad which lets you use your other music apps together. Chain effects on your favourite synth, run the output of apps or Audio Units into an app like GarageBand or Loopy, or select a different audio interface output for each app. Route MIDI between apps — drive a synth from a MIDI sequencer, or add an arpeggiator to your MIDI keyboard — or sync with your external MIDI gear. And control your entire setup from a MIDI controller.Download on the App Store
Audiobus is the app that makes the rest of your setup better.
Pushing the envelope
In the interests of being an awkward cuss and stretching things further than they're supposed to go...
Say I have an instrument on my iPad which isn't really playable on my iPod, and I want to play it out through a non-Audiobus fx app on my iPod and take it back into Audiobus on the iPad to record into one of the Audiobus recording apps. I would have to a) use two iRigs or similar, b) split the headphone out from my iPod so I can hear what I'm playing and c) be able to mute the recording app so that only my instrument app presents audio to the iPad output.
Anyone think of a good reason why that wouldn't be possible?