Correct me if I am wrong but when I purchase Audition, I believe it comes with 2 copies. Yes?
In Adobe language, when you purchase or subscribe to the software, you get one license. One license can be activated for use on two separate computers.
The exact rules surrounding the activations depend on which version you buy.
With Audition CS6, you buy a perpetual license (pay once, use forever) and get a platform-specific serial number which can only be used on 2 x Macs OR 2 x Windows. You cannot use the same serial number on both Mac and Windows.
With Audition CC and CC 2014 (Cloud) subscription versions, you can activate the software (via your Adobe ID - there is no serial number) on both Mac and/or Windows for no extra cost - i.e. Mac/Mac or Win//Mac or Win/Win.