Varying drive

The DRIVE control has too many calculations associated with it to allow a modifier to

Neither technique is going to be quite the same as simply adjusting a volume pedal to shift the gain. Panning between the inputs would better approximate this, but you're not really gaining much (pardon the unintended pun) by using a second setup then. For blending to a different tonality (such as boosted mids and smoother gain) for a solo though, that's a great approach.

Mixing between the outputs can be a nice effect, and useful for other things, but really isn't anywhere near the same as adjusting a gain control. If you mix between a signal and a version that's been pitch-shifted by an octave, you don't get a shift of a fifth. Similarly, you won't affect gain structure by mixing between low gain and high gain. It's a great way to clean up a really dirty sound to get more clarity, but not the same as turning down the distortion.

To think through this stuff, just think about what you'd get if one channel was set to crystal clean and the other was a raging fuzz. Panning the inputs would simply fade out the clean as you increase the drive on the fuzz, but you'd still have a blend in the middle of the spectrum. (Again, not bad, just not what you're asking.) Mixing the outputs, you would just get more pick attack sticking through the fuzz. (Still useful; still not what you're asking.)

A volume pedal at the input of a Drive or Amp block is the way to go. You can set a range in the modifier so it varies between two set points. You can also throw a control at a LEVEL control somewhere further down the line (or another Volume block) to counteract (or further, if that's what you want) the volume boost from pushing everything further into saturation.