In reversi, can you overwrite two chips in one move?
No. From the Rules of Play:



  Each piece played must be laid adjacent to an opponent's piece


  so that the opponent's piece or a row of opponent's pieces is flanked by the new piece and another piece of the player's colour.


  All of the opponent's pieces between these two pieces are 'captured' and turned over to match the player's colour. 



where I have split up and numbered the clauses of the paragraph for easier reference.

Note that:


from (1), the suggested move is illegal, not being adjacent to an opposing piece; 
even if one is playing to make -xxyxy-- into -xxyxyx-, by (3) only the second 'y' is flipped, resulting in -xxyxxx- upon completion of the move; and
playing to make -xxxyy-- into -xxxyyx- does convert multiple opposing counters, by (2) and (3), flipping to yield -xxxxxx- upon completion of the move.  

Reason:

"All of the opponent's pieces between the these two pieces [(the just placed piece and the enabling friendly piece at the far end of the just surrounded enemy row of pieces)] are 'captured' and turned over to match the player's colour."