Longest sequence with only moves to draw without a repetition What is the longest known sequence of moves with upto 6 pieces where each move of
 both sides is the only move drawing the game (any other move would lose the game) ?

There should not occur any repitition of a position.

I experimented with the endgame rook vs two (three) pawns and got some
 sequences I unfortunately do not remember.
To get the ball rolling, from the following position the first 6 moves for each side are forced in order not to lose. (If 7.h8=B weren't available and 7.h8=Q needed to be played, then the forced follow-ups 7...h1=Q+ 8.Qxa1+ Kxa1 would extend it to 8, but no cigar.)

[FEN "K5R1/8/p7/8/8/7P/8/1q1k4 w - - 0 1"]

1.Rg1+ $1 Kc2 $1 2.Rxb1 $1 Kxb1 $1 3.h4 $1 a5 $1 4.h5 $1 a4 $1 5.h6 $1 a3 
$1 6.h7 $1 a2 $1 7.h8=Q 
    ( 7.h8=B )
7...a1=Q+ $1 8.Qxa1+ $1 Kxa1 $1 *