Given multiple admin accounts, how can I make it so that only admin with X username can edit posts

Once you say “edit” I think you are no worried about new posts, just edit existent post.

So, my idea is remove the support fot title when the admin is no the one allowed.

I’ll write a separate function that retturn true if the current user is the wanted admin, in this way it can be used in different part of the site.

function is_preferred_admin() {
  $u = wp_get_current_user();
  // change 'admin' with the wanted login here
  return user_can($u, 'manage_options') && $u->user_login === 'admin';
}

add_action('load-post.php', 'remove_title_support');

function remove_title_support() {
  if ( is_preferred_admin() ) return;
  $scr = get_current_screen();
  remove_post_type_support( $scr->post_type, 'title' );
}

Consider that this remove UI, but is not a real capability preventing. To do that, filter wp_insert_post_data and prevent title changing:

add_filter('wp_insert_post_data', 'prevent_edit_title', 999, 2);

function prevent_edit_title( $new, $oldarr ) {
  if( is_preferred_admin() ) return;
  if ( ! isset($oldarr['ID']) || empty($oldarr['ID'])  ) return;
  $old = get_post($oldarr['ID']);
  $new['post_title'] = $old->post_title; // no change allowed
  $new['post_name'] = $old->post_name; // no change allowed
  return $new;
}