you can use:
if ( in_category('cat1') || in_category('cat2')) {
include 'single-cattwo.php'; //for the 2 categories
} else {
// Default
include 'single-allcat.php';//all categories
}
inside single.php
you can use:
if ( in_category('cat1') || in_category('cat2')) {
include 'single-cattwo.php'; //for the 2 categories
} else {
// Default
include 'single-allcat.php';//all categories
}
inside single.php