You can do it all with one, but it takes a little routing and programming if you do. Typically, one would use the multiplexer with the volume and wah in two parallel rows, both attached to the same pedal as a modifier. Your switch to turn on the wah would actually be a multiplexer channel selection.
Next step would be 2 expressions, one for wah and one for volume. No routing/programming/multiplexer needed.
Then you get to wanting an expression for things like delay time or feedback, whammy/pitch shifting, modulation speed, or other special effects (e.g. multiple parameters linked to the same expression in a specific program). Then you might want more than 2, need to do the multiplexer thing with different blocks, or some midi-controller trickery.
A Ben.Last pointed out, sometimes it's useful to have different types of expressions for different purposes. I really like a long throw for volume to get fine nuance than a typical crybaby throw. But I've come to realize that some wah techniques (e.g. fast pseudo-vibrato) work best with a traditional wah throw.