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 StoreAudiobus is the app that makes the rest of your setup better.
Comments
Which iPhone? And does the SP404 have a line out? Maybe you can just run a cable from one to the other? (With a Lightning > jack dongle on one end).
Yes, it does. Tried the method above and it doesn’t seem to work - it just recorded Microphone audio when using the camera.
iPhone XS Max.
I haven't tried it but this app seems to support external audio interfaces:
https://apps.apple.com/de/app/promovie-recorder/id1076481548?l=en
Always reliable to record both separately
Record your playing on iPhone and connect your keyboard to software
Once done just replace the audio from iPhone video with a polished one from software or App