Motive? Dont blunder.
2 knights vs Rook
In most endgames like this, the side with two pieces has the advantage if they can combine.
It seems like you played with a reasonably clear plan - linking your knights to protect your a-pawn, then bringing the king over. Then he shortened your task by falling to the fork. Nevertheless, you had the advantage here. To progress (after say 47.Re1) the best approach would just involve continuing to gradually manevoure around, driving White back and the a-pawn forward.
It should be a draw on your side IF all the pawns get off the board. Also if it is a pawn for your opponent this is still probs a draw I hope this helps and Hi chessnetwork :)
I recently had a game where I had two knights against his rook in the endgame: http://www.chess.com/echess/game?id=116032650. That game had a couple sweet tactical shots, one by him to win my rook and two pawns for his knights, and one by me to win a pawn and blow up his king's cover.
My knights were better, but I ran out of ways to make progress, and it ended up a draw.
"The principle of redundancy" apply for the two knights.
"...At the same time, two knights frequently face problems fighting against a rook. The rook has a much more complicated task when playing against a knight and a bishop."
I recently got a 2 knight +5pawn vs rook + 4 pawn endgame and won that game because he missed a fork just wanted to enquire what should be my motive in any 2 minor peice vs rook endgames.