There's a world of difference between multiple busses - which are virtual, and only exist in software - and multiple hardware outputs, I'm afraid. Like, no actual relationship at all...
Audition has an ASIO engine, and one of the basic conditions of ASIO is that you can only use one physical device with it. This usually works fine, because you can get multi-channel sound devices, which pretty much satisfy most people's reqirements. That said, what you want to do may be possible, but will need to be set up carefully. On top of that, Windows really doesn't want you to use more than one internal output at a time.
Are there solutions? Possibly, but no guarantees. Initially, what you have to try is to use a different output driver system, and there's basically only one that will work with Windows. Fortunately it's free - it's called ASIO4ALL. What it does is to provide a virtual interface between Audition's ASIO engine and whatever hardware you happen to have attached to your computer, and it bypasses most of the layers of the Operating System. To use it you have to alter your Audition device class to ASIO, and then it should be listed. It's worth doing this anyway, because ASIO4ALL has excellent diagnostics, and will tell you at a glance what it's actually possible to connect to.
You can find it here: ASIO4ALL but I must reiterate that there are no guarantees. The only other possibility is more expensive and seriously complicated - it's called Virtual Audio Cable, and essentially does a virtual rewire of your computer's innards. I really wouldn't go there unless you are desperate. I believe that there's also another program from the same stable called Voicemeeter that may be easier to set up.
But, you are fighting the OS - Microsoft really doesn't want you to do this. The only really effective solution is to get a multi-output sound device with an ASIO driver, and have all your outputs from the single device. This has the advantage of getting around all of the synchronisation problems (yes there are some) that you have with multiple hardware devices.
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