I just came across a new resource on github called BlackHole which is a modern replacement for soundflower or the commercial LoopBack program as far as feeding the output of soft synths such as helm or qsynth running on a Mac back into Sonic Pi as live_audio inputs. You install BlackHole (installer provided, but as for current Sonic Pi installations use right click and open rather than double click to run). Once installed you can select it as the default input from audio MIDI setup and then Sonic Pi will use it as an input source and you can select it in helm or qsynth as the destination for their audio output. qsynth only lets you specify input name not channel, and seems to default to channels 1 and 2. However Helm lets you specify channels 3 and 4, so you can run both with independent audio in using
live_audio :qs, input: 1,stereo: true and live_audio :helm input: 3,stereo: true which works fine.
BlackHole will also be useful as part of an aggregate device, and there is a wiki on the github site with useful information.