We're working on a complete replacement of Stereo Tool's input/output mechanism. This will give a more flexible system, in which something like a dual output will likely be possible. It will take a while before it's finished though.
For now you could use something like VBCable or Virtual Audio Cable, but that will cost you latency unfortunately.
I hope that will be soon, since 2nd (current) version is still not good at some aspects. It would be good after all this years atleast that part be fine.
I am testing this buffer synchronisation (notmal output) and it works totally wrong. For starters, measurement of buffer position (M) is constantly in delay. 10 to almost 20sec, i guess depends on Buffer Size, SampleRate.. etc, instead in realtime like buffer meter, for example.
However it's not only I/O part wrong, something in ST core is also acting wrong, since changing "Quality" to other number then 100% changes things a lot.
What crosses my mind atm is that you can temporarily use "Leif's Resampler" or VAC's repeater so you can pass ST's Normal Output to 2nd card. Rasampler would be my choice, it works very good.