if you purchased an audition 2 to cs6 upgrade, you should be able to install audition cs6 using its serial number and audition 2's serial number.
it's not necessary for audition 2 to be installed.
the only issue is, i don't see an audition 2 to audition cs6 upgrade path. i see an audition 3 to audition cs6 upgrade path: http://www.adobe.com/products/audition/buying-guide-upgrades.html
Thankyou for your response, kglad.
I am peering more intently at the serial number dialogue box, and now I understand what you mean - it's asking me for the OLD product and SN , before allowing me to use the new. Excellent, thanks!