Normally, when I see something like "C/C++" it means that there's a significant overlap between the two languages, so knowledge of one implies or coincides with knowledge of the other. But C# and C++ are very different languages, and while you could argue that there's some sense in which they're related, it isn't much of one. If you're going to list "C#/C++" then you could just as well put "Java/C#", or "C++/Rust".
Normally, when I see something like "C/C++" it means that there's a significant overlap between the two languages, so knowledge of one implies or coincides with knowledge of the other. But C# and C++ are very different languages, and while you could argue that there's some sense in which they're related, it isn't much of one. If you're going to list "C#/C++" then you could just as well put "Java/C#", or "C++/Rust".
You could reasonably use C/Rust and C/Haskell to say that you have mastered the fields of
unsafeandGHC.Prim.C#, though? C#/Java would make sense given that they're the same language with slightly different Syntax.