How do I find the relationship between 2 individuals in FamilySearch, both of which are not in my personal Tree?
Go to Relative Finder at https://www.relativefinder.org/#/main
Sign in. You use your FamilySearch account. After going through the authorization process it will download a section of Family Tree starting with you. This is not needed for what you want to do, but it does it. After that you will come to a page for exploring your relatives. However, over the on the upper right, under the main banner is a set of menus. Use the Connect menu to find the "Two Deceased People" option:
That will bring to a page where you can put in the ID numbers for the two people.
Enter the IDs, click submit, and it will tell you the relationship.
Many thanks. This was exactly what I needed.