The short answer is that you cannot think of USB audio the same way as analog audio interconnects. It’s made for computer connectivity.
I think your options are:
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Connect both devices to a computer running Overbridge.
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Connect all devices to a computer (or tablet/phone) via a 5+ channel audio interface (analog).
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Connect all devices to an audio mixer (analog).
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Daisy-chain audio through the analog inputs on the devices themselves.
It is only possible to do the USB audio routing you want when both devices are connected to a computer running Overbridge in a DAW.
Overbridge enables this kind of connectivity because it uses a custom driver instead of class-compliant audio.
It’s technically possible to use an aggregate device on a computer with class-compliant audio as well, so that a DAW sees the two boxes as a single interface; but that is far more trouble than it’s worth in my opinion (it’s a last-resort for devices that do not have Overbridge-like connectivity).
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With both the Digitone and Rytm having analog audio inputs/outputs, you should be able to connect all three devices together though.
RE-303 into Rytm, Rytm into Digitone.
That is by far the easiest option (though it’s not going to scale well beyond that).
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If you want more flexible audio routing/mixing, you would need an audio mixer with 5+ inputs (which mixes analog audio signals).
Or to use a class-compliant multi-channel audio interface connected to a device like an iPad - if you don’t want something as big as a mixer, but also don’t want to deal with a “computer” or traditional DAW.
The advantage of this setup (or even daisy-chaining as above, and then connecting the Rytm or Digitone via USB) is that you can now bring software instruments or effects into the mix.
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The one exception to how USB audio typically works is the Roland Aira MX-1 mixer.
This mixer does have USB Host ports for multiple devices; but they are only compatible with Aira Link devices for bringing USB audio into the mixer, powering the device, and MIDI/Transport connectivity over a single cable.
You cannot connect regular class-compliant devices to these ports.
I do think there’s a market for a digital mixer that has its own class-compliant host ports on the back - or better yet, for Elektron to copy the MX-1 but have Overbridge-enabled host ports on the back.
But I don’t believe any kind of device like this currently exists - probably because it would be a massive undertaking to ensure the class-compliant audio works reliably with all devices, and the benefit over using a regular mixer is arguably going to be quite small.
