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.
There were some posts there a while back about things window users have to remove and some thing to install that solved a lot of problems for people using it on Windows. I’m using Mac . So, can’t help.
I solved this using Studiomux, and a negative track delay on the source track in Ableton to compensate for the ~35 millisecond round trip. This uses a single USB cable, and can send several tracks over the connection, in both directions.
Now I've got my head around how it works, it's a pretty great setup.
I have a reasonable outcome in a somewhat similar senario. iConnectM4+ aggregate with MOTU M4. 2 iPads into MacBook running Renoise. I'm able to run audio and midi any direction between all 3 devices. I generally am just using it to process samples , iOS midi processors, and plugins that are a way better deal on iOS though. The audio latency is only about 10-20 ms. There are occasionally some hiccups or dropouts but for my purposes it's not frequent enough to be a problem.
New solution: I ran cables from an output to an input on my K-Mix interface for a hardware loopback. Then I can use the loopback app to send audio out the correct channel.
Works a treat, and free, too! And no added latency from aggregating devices.
One general tip I'd add - if you're connecting to the Mac via Lightning or USB-C - I've had best results if the Mac is connected via Ethernet - then use the Internet Sharing option in Sys Prefs - then you can turn off Bluetooth & Wi-Fi on the iPad- still get all the functionality... IDAM, AirDrop, Internet, Sidecar & Universal Control. This has worked best for me w/ latency - YMMV!
Also, I've seen here a few times that IDAM is 44.1 - it's supposed to be 48, but I think this might be a bug as some devices don't support certain sample rates. Also, make sure the 1st device in an Aggregate can be a Leader, if it can't, then either make it a secondary device, or set up some drift compensation. I've found Loopback to be much more reliable though than aggregates. ~Best!