How do you add a union skin to a single part on a sheet that has other parts on that sheet with normal through routing.

Say the sheet has 10 regular parts on the sheet of .75 in depth . I need to have two small parts on that same sheet to have a union skin. How do I do this? I have decreased the depth bit to cut the small parts to .6875 but the bit will still cut a .75 depth leaving no union skin.