iOS music making apps no longer exist as islands, but connecting them all together in the way that you want and mixing their outputs isn't always straightforward.
Enter Kymatica's AUM, "an audio mixer, recorder and connection hub" for iOS devices. It enables you to connect hardware inputs and outputs, AU extensions, Inter-App Audio and Audiobus apps and more as you fancy. Indeed, the pitch is that this is an 'anything to anywhere' solution, with support for mix busses, effect sends, Audiobus ports and external multi-channel audio interfaces.
You can also play back hosted synths from your MIDI controller or the built-in keyboard, or sequence them from other apps using virtual MIDI.
You can get a flavour of AUM's routing options in the video above, and it can be purchased for £7.99/$9.99 from the Apple App Store.