deriv LSK ETT STT aSTA ALPH OLDHOMEPAGE NEWHOMEPAGE

/au / औ

/au is the dual /sup ending of the first line /svaujas, and of the second line /amauTchaS.

**azva- अश्वॱ + /au → **azvau अश्वौ "two horses"

These two /au are the exact same affix, so there is never any difference between the first dual —

azvau dravataH अश्वौ द्रवतः "two horses run"

and the second dual —

azvau pazyAmi अश्वौ पश्यामि "two horses I see"

Isn't the one in /amauTchaS, the second line, an auT औट् affix, with a T ट् |labelletter?

No, it is not. That T ट् is there solely to stop the /suT' /pratyAhAra. It is not part of the affix. The second dual ending is /au, not auT औट्.