But the standards have made some effort at avoiding gratuitous incompatibility, and have even introduced changes to reflect changes in the other standard.

Also, much C code is still valid C++. Sure, you can write code that isn't, but I would guess (pulls a number out of nowhere) that 90% of the valid C code is also valid C++.

That makes them languages that have separate standards, but not completely independent standards, that share a whole lot of source code. That's something less than fully separate in my book.