Si solo vas a usar 2 opciones:
Código PHP:
$pattern = "123456";
$pattern6 = "12345"; //Sin 6
$pattern5 = "12346"; //Sin 5
$pattern4 = "12356"; //Sin 4
$pattern3 = "12456"; //Sin 3
$pattern2 = "13456"; //Sin 2
$pattern1 = "23456"; //Sin 1
$opcion1 = $pattern1{rand(0,5)};
$opcion2 = ${"pattern".$opcion1}{rand(0,4)};