You can use TimeFactor (or PitchFactor) as a master volume controller:
- Turn Killdry to YES in the System/Bypass/Killdry menu.
- Turn Delays, Filters, and Feedback to 0. On PitchFactor set Pitches to Unison.
- Assign Expression Pedal to control the Mix knob.
You could also expermiment with various settings of the Dly Mix knob (TimeFactor) or Pitch Mix knob (PitchFactor.) This way you could use one effect engine as your 'dry' signal and ther other as your effected signal, changing the overall volume with an expression pedal assigned to Mix knob.