The two atoms are not required to be bonded.
Both atoms should be addressable i.e. listed in the primary atom list as shown with the LIST ATOMS instruction. When a clicked atom is not yet in this list, it will be added automatically. The added atoms are now available for distance calculation (i.e. reenter the DIST instruction) and their label shown.
Note: No s.u.'s are available with this option. Use the ORTEP link instead when s.u.'s on bonds are needed as well.