Varying drive

The DRIVE control has too many calculations associated with it to allow a modifier to adjust it in real time. You can adjust gain in a preset in a few different ways, each with their own benefits and drawbacks. Recreating the effect of varying the DRIVE control has a certain set of needs that other techniques won't achieve.

Parallel paths: panning between inputs or mixing between outputs

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 isn't much gained (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 mixing between a signal and a version that's been pitch-shifted by an octave doesn't get a shift of a fifth. Similarly, gain structure isn't affected 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 would happen if one channel was set to crystal clean and the other was a raging fuzz. Panning the inputs would simply fade out the clean while increasing the drive on the fuzz, but there would still be just a blend in the middle of the spectrum. (Again, not bad, just not riding the DRIVE control.) Mixing the outputs, you would just get more pick attack sticking through the fuzz. (Still useful; still not a variable DRIVE control.)

Volume block before the gain block

A volume (or null filter) block 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.