why dont you use a repeater in each CPT called something like ‘relational meta’ and acf update_field.
you can use wp noonce ajax to take data from the first CPT, pass it to your function (in functions.php) with ajax check if the second CPT exists, then check if the second CPT has the data you want to save, if not, save it. This is also benifical as you can detach your logic from wp concerning post meta and use ACF as your conduit.
an added bonus is this is all done without reloading the page.
this would be a mix of wp, acf, js and ajax. Ping if you require further assistance.
