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
Line 6 sonic port is pretty good. The vx model also has a built in microphone and can charge the iPad if it’s powered
The Roland Go is made for doing this.
What iPad do you have? Lightning port?
Maybe these?
https://www.amazon.com/Behringer-U-Control-UCA202-Ultra-Low-Interface/dp/B000KW2YEI/ref=sr_1_1?keywords=Uca202&qid=1553258891&s=gateway&sr=8-1
https://www.amazon.com/Cablecreation-2-Male-Splitter-Design-Stereo/dp/B06XFTHCNY/ref=sr_1_12?keywords=Headphone+to+stereo+splitter+rca&qid=1553258949&s=gateway&sr=8-12
https://www.amazon.com/Apple-MD821AM-Lightning-Camera-Adapter/dp/B014VGHG0U/ref=sr_1_4?crid=1O3BEL3XPISQS&keywords=apple+camera+connection+kit&qid=1553259032&s=gateway&sprefix=Camera+connect,aps,188&sr=8-4
Maybe there is something better now though but in my mind if you only want to record the headphone out then I would not spend much on the recording device. The best quality would be two audio interfaces or an audio interface that allows two devices to be hooked up to it... I think there is one but the name escapes me. Although maybe that one was iOS/desktop and not iOS/iOS?