While not as versatile as the Aux output, this workaround may meet your needs:
In AudioFuse Control Center, turn on "SPK B" button under "Phones 2". Create a CUE mix that only contains whatever you want to output to speaker B (for example, USB 5-6), and assign this mix to Phones 2. Now you will get USB 5-6 as output on Speaker 2 (while being able to have different mix on Speaker 1). You will lose ability to plug headphones in Phones 2 output.