I use two usb audio interfaces in Rack on a mac simultaneously, but the only way I’ve been able to keep them from drifting and distorting periodically is to create an ‘aggregate audio device’, which is something that macOS supports. Then I can engage ‘Drift Correction’ on one, and I believe it effectively slaves the audio timing from one interface to the other somehow.
From Rack’s perspective they end up being one audio device. Ever since Rack 2.0, I can select two different audio devices within Rack, but I can’t seem to prevent their timing from drifting and causing clicks.
Here’s a picture of what I mean by the Aggregate Device:
Anyway, it sounds like you might be working in Linux? So this probably isn’t directly helpful, but this is my experience of running two audio devices on macOS. It is very consistent and useful, but it would be much simpler if I could just select two different devices at the same time and not worry about it.