> And the manual says you can use multiple audio devices from the
> settings,

I think that's true, and here is my quick test.

I connected a second soundcard to my laptop so I have one built-in and
one USB.

In Pure Data (self-built 0.52.1 with some irrelevant local hackery),
Menu -> Media -> Audio Settings... gives me the usual Audio Settings
dialog (sample rate, delay, input & output soundcard dropdowns ...) and
a button "Use Multiple Devices".

Clicking that button opens another dialog which looks almost the same,
but the "Input Devices" and "Output Devices" sections now have multiple
rows of checkboxes and dropdowns. Now I can select one soundcard for row
1, the other for row 2, assign the number of channels on each device
(like in the single-soundcard settings menu)

The help on [adc~] / [dac~] explains how to use the additional channels.

Best regards,
   Albert.