[IstuntmanI]

Use 'random' function and a 'switch' :

Код:

if( strcmp( cmdtext, "/coin", cmdtext, true ) == 0 ) 
{ 
	new str[ 128 ]; 
	GetPlayerName( playerid, str, MAX_PLAYER_NAME );
	switch( random( 2 ) )
	{
		case 0: format( str, 128, "%s flips the coin and lands it on heads", str );
		case 1: format( str, 128, "%s flips the coin and lands it on tails", str );
	}
	SendClientMessageToAll( 0x0780E3FF, str ); 
	return 1; 
}

( I also improoved your code )