I apologize if this has already been covered in this thread, but I thought this might be of use to others. Lately, I've been back to gigging, in some noisy, problematic environments. I contacted Grace Designs to see if it was possible to bridge the two channels, so I could use the HPF and notch filter simultaneously. Turns out there are a few different ways, this is the one that I used. Like many things Felix, it is rather complicated!
1. Plug into channel 1 (this is to optimize the 10M impedance for my Yamahiko pickup)
2. Use the insert out to feed channel 2
3. Turn mix knob to channel 1
4. Use the amp out (sourced to channel 2) to feed amp
Hope this helps someone in a similar situation, and if this has been shared before, feel free to delete.