Those "two satoshis" may be indistinguishable, but they are inexorably linked by their common tx.  Hence, the anti-taint cottage industry of mixers and tumblers and shufflers and Darkcoins (lol).  And (more significantly) the booming trade in blockchain analytics.

Let's set aside considerations of fungibility at the recipient level (maybe I'm sending BTC to MPEX, where they don't GAF where my BTC has been).  There is still a privacy-driven fungibility issue for the sender.

IE, a confidential transaction is not, from my sender POV, equal to a public one.  Maybe I don't want my ex-wife/IRS/boss to know where I'm sending BTC.  The public and private tx are not interchangeable, and that's why people pay proportionally extra (directly and in the form of TTP risk) for fresh coins and mixed/joined/tumbled/shuffled ones.