I purchased a Behringer PRO800 synth (really interesting) and I would like to use Zynthian as effects unit
The built-in sound card in my custom Vangelis Zynthian does not have audio in connectors but I have a USB sound card available with audio in
now the question:
is it possible to use two sound cards simultaneously?
specifically, one for output and one for input?
how should I do it?
If you use a raspberry pi 5, you can engage hotplug usb audio from the admin menu.
A pi4 also has this switch, but is not capable enough to handle the resampling which is needed with multiple soundcards.
Also, I read that
In addition, the Behringer Pro-1 has a USB port that allows it to be accessed by modern DAWs.
So probably, you can just plug in the behringer directly as a soundcard. Still needs the hotplug setting though.
Is there a reason you want both soundcards or could you just use the USB card for both input and output?
main soundcard in my custom Zynthian is always connected to mixer
USB soundcard would be used only as audio in
yes my Zynthian is RPi5 based
good suggestion, but unfortunatelly PRO-800 cannot send audio over USB (MIDI only)
Sorry, confusing search results, the pro-1 has it, the pro800 does not. The usb is only for midi.
Yes but why don’t you use it for both and abandon the direct connected, output only soundcard?
Wait, HotPlug means you can use both interfaces at once? I thought that was impossible with alsa at the bottom, the linux version of dividing your audio by zero?
If so, good to know. I did notice, doing my midi work this week, that Pipewire puts everything in its qpwgraph but I haven’t done any testing on latencies and whatnot, I still assume I’ll be using jack or pipewire-jack when I’m doing audio work/processing.
to keep the main connection to mixer (only for clean and lazy reasons)
In that case I would recommend ditching the internal soundcard and using the external one only. It solves all your problems and only requires you move a couple of jacks. 
Yes - that is exactly what hotplug provides. There is an overhead as mentioned above. You have the primary interface acting as the jack master clock. All other interfaces are connected using alsa_in and alsa_out which are jack clients, sampelrate converting the ASLA device to match the jack clock. (Before we heat the same arguement yet again: both cards running at 48000 is not synchronised. One may run at 48001 fps and the other at 47999 fps which means the buffers will peridocally empty or overflow and click.) We do not recommend using hotplug USB audio but it does work, especially on RPi5 and is an option if ti suits your workflow.
This is true. Pipewire effectively includes the alsa_in/out functionality, samperate converting all ancillary soundcards so they appear in the graph. We have exherted some effort trying to get Pipewire to work in zynthian but there are some challenges that have yet to be overcome. We also haven’t done the benchmaking to see if it is better, worse of the same as jack for overhead. It offers a lot of extra features but most relate to desktop environments and its low-latency “pro” audio is less developed than other bits and may still lag jack. We may revisit again soon. The hurdle that we couldn’t resolve was the refusal to run some modules as root.
I’ll be very interested to know those results! In theory since PW can emulate jack you would assume it is as fast as jack natively, or possibly even better, but you’d also need native pipewire programs, probably.
Hi Paolo,
As you may have read in another thread, I am also using Zynthian (very successfully) to stereo-process with FX the mono output of a Pro-800.
With your specific setup, I would advise too to go for a single external USB audio board, if this is compatible with the physical layout of your DIY setup.
I was previously using my old V4 as a flexible FX unit, but managed to damage one of its audio inputs with poor soldering (my bad), and now am processing my Pro-800s through the V5, because I need two mono inputs.
Best regards 
For a fast-easy-lazy solution I will use my V4 Zynthian upgraded with RPi5
It has audio in so it is really a piece of cake