On re-reading the original post it looks like this menu has multiple list items with a choice of classes.
Currently my navigation bar is split into 2 css classes, menu and customer. The “menu” section has a menu separator in orange and the “customer” one has a menu separator in dark grey.
What the OP seems to be after is, if I understand correctly, a “nth-of-class” selector which, as we know, does not exist.
Frankly, it seems more logical to me to use two menus next to each other each with their own class and styling.
From the visitor’s perspective they could easily appear to be one menu so that’s not really an issue.