There have been many articles lately about inter app audio and Audiobus 2.0 on the iPad and I just wanted to put in my two cents.
As much as I enjoy creating music on the iPad, the one area that still feels a bit "primitive" is getting apps to play nicely together. Much of this is due to Apple's app "sandboxing" but another aspect is the maturity of the whole infrastructure.
Audiobus arose more or less "organically" outside of Apple and managed to provide a much needed but still somewhat limited means of combining apps musically. Up to 3 sources can all pipe through effects and into one target. I believe that this introduces a degree of latency, but it can be handled as long as all items are going through the same path.
IAA (Inter app audio) is Apple's solution to combining musical apps and uses internal API items not accessible to 3rd parties - not exactly fair but who's counting. IAA works with much less latency and you can concurrently use as many apps as your CPU can handle (not that many really!).
The exciting news is the upcoming Audiobus 2 which will actually save your plugin settings (assuming that the source applications support the feature). This is slowly getting to where you almost use the iPad like a desktop DAW.
The big difference on the iPad is that you are constantly worrying about the plumbing! How many apps are open? What presets need to be in each synth to combine my music?
This happens quite a bit in my own creations with Noatikl where each synth must be played and captured at the same time (since Noatikl has each voice harmonize and play off the others in real time). I have experimented with both and once tried mixing and matching both IAA connections with Audiobus at the same time. They don't mix! Latencies are just off enough that the resulting files were way off time-wise and it was too much work to finagle them in Auria after the fact.
This means that for my Noatikl pieces that need many concurrent synths, they all need to be Audiobus (limit - 3!), or they all need to be IAA.
When back on my desktop using Logic Pro, I never worry about the AU connections - they just work. If I save my song, all the settings are retained, there is rarely a CPU issue and I concentrate more on the creative process than on plumbing.
As things stand now on the iPad, I'm still feeling like Super Mario at times!