Related field sort order

I have two custom post types (not extensions of existing types). cpt_one and cpt_two. When I edit one of my cpt_one records, I want it to list out cpt_two in checkboxes sorted by custom field sort_order defined in cpt_two. I cannot figure out how to do this at all without PHP. In my cpt_one definition, in the custom Order By field, I put “sort_order ASC”. I’ve also tried {@sort_order}, t.sort_order, d.sort_order. I can’t get it to sort by anything other than alphabetically by cpt_two’s title.