sort posts by a sum of two metakey values

The only option I can think of is to create a hidden ‘sum’ meta field that is automatically updated when the post is updated and then sort on that.

There are a couple of action hooks you could use: update_post_meta & updated_post_meta which fire before and after updating the meta values. I would take a bit of coding but it should be hook into those and update the third ‘sum’ field when either of the other two are updated.

Look in wp-includes/meta.php at the update_metadata function to get an idea of how those hooks work.

You can do it using filters on the query:

The filters can go in functions.php or in the template ahead of the query (I would put them in the template at least while testing).

function mam_posts_fields ($fields) {
   global $mam_global_fields;
   if ($mam_global_fields) $fields .= ", $mam_global_fields";
   return $fields;
}
function mam_posts_join ($join) {
   global $mam_global_join;
   if ($mam_global_join) $join .= " $mam_global_join";
   return $join;
}
function mam_posts_orderby ($orderby) {
   global $mam_global_orderby;
   if ($mam_global_orderby) $orderby = $mam_global_orderby;
   return $orderby;
}
add_filter('posts_fields','mam_posts_fields');
add_filter('posts_join','mam_posts_join');
add_filter('posts_orderby','mam_posts_orderby');

This part has to go in the template:

$mam_global_fields = 'SUM(metaA.meta_value, metaB.meta_value) as meta_sum';
$mam_global_join = "JOIN $wpdb->postmeta metaA ON (metaA.post_id = $wpdb->posts.ID AND metaA.meta_key = 'metaA')
   JOIN $wpdb->postmeta metaB ON (metaB.post_id = $wpdb->posts.ID AND metaB.meta_key = 'metaB')";
$mam_global_orderby = " meta_sum ASC";
// Your query_posts goes here