wellll...
since I've only got one pair of biking shorts, and a million and a half sets of coolmax/drifit/whatever underwear, I just wear the underwear under the shorts and bring a spare pair for thru the day/ride home... It doesn't cause me any chafing issues to do so.

I probably wouldn't worry, I'm certainly not a neat freak, but I have a vague fear that my chamois would get sort of... uncomfortable being used 2x with nothing between me and it... and only one pair begs me to reuse as often as possible between washes. why do they have to be so expensive?!