[M4S7ERMIND]

You could make a variable to store ids of challenger and challenged.

pawn Код:

new Challenge[MAX_PLAYERS] = { INVALID_PLAYER_ID, ... };

Once challenger challenges the challenged:

pawn Код:

Challenge[playerid] = challengedid;
Challenge[challengedid] = playerid;

If challenged clicks YES:

pawn Код:

if(Challenge[Challenge[playerid]] == playerid) //This checks if challenger hasnt challenged someone else while challenged was thinking
{
  SetPlayerPos(playerid, x, y, z);
  SetPlayerPos(Challenge[playerid], x, y, z);
  //When challenge ends, set both players Challenge to INVALID_PLAYER_ID;
}
else
{
  SendClientMessage(playerid, WHITE, "This player has challenged someone while you were thinking.");
  Challenge[playerid] = INVALID_PLAYER_ID;
}

if challenged clicks NO:

pawn Код:

if(Challenge[Challenge[playerid]] == playerid)
{
  SendClientMessage(Challenge[playerid], WHITE, "Challenge cancelled..");
  Challenge[Challenge[playerid]] = INVALID_PLAYER_ID;
  Challenge[playerid] = INVALID_PLAYER_ID;
}
else
{
  Challenge[playerid] = INVALID_PLAYER_ID;
}

Well you get the idea..