OK, I have two work-around suggestions for you:
Approach 1
Keep the two sets of credentials in separate login items, but set the URI match detection for the back end login to “Never” (or simply delete the URI). To autofill the back end, search for the corresponding vault item (you may want to tailor the item name or use other SEO tricks to make it easier to search for), then open it and click the “Autofill” option from there (in the redesigned browser extension you will be able to autofill directly from the search results list, no need to open the item).
Approach 2
Us a single vault item, but define custom fields to hold the login credentials for the back end login form. By making the custom field names match one of the identifying attributes for the login form input fields, you should be able to autofill the back end login form as usual.
Let me know if one of these works for you!