Listing unusual dual character roles for tv episode.

In a TV series, each episode credits the character names in the regular opening credits. Sometimes a character doesn't appear; IMDb has the attribute "credit only" for this. One particular episode has some actors playing two different characters. These would list both characters on IMDb separated by a slash.

In a few cases, though, actors are playing only the additional character, and not their regular role, even though they're credited for the regular character in the opening credits. Should both characters still be listed (separated by a slash)? In that case, how do we specify the attribute (credit only) for just that regular character and not the other character?