Is Sphinx already suitable for C++ documentation?

For C++ Doxygen is very good; Sphinx support of C++ is done via Doxygen (using Breathe).

You want to have only one tool for the two languages, to improve maintainability. Doxygen can generate a documentation from Python code, so I advise you to use Doxygen for both sources, if possible. Here is a tutorial for Doxygen and Python.

Sphinx cannot extract documentation from C++ sources in and by itself. However, there are extensions, most notably Breathe, which utilize Doxygen to extract documentation from C++. I've not tested any of these.