You just need a mixer with auxilliary outputs. For example, I've got an A&H Zed14, which has four aux outputs. I route two of them to the stereo inputs on my audio interface. Then I can dial in which ever channel on the mixer I want to send to the aux outputs (mixing mutiple synths), which are then sent to PC (mixed with the PC audio), then sent back to the mixer and then to the monitors. There's no feedback loop, since the output from the PC never goes back into the PC.