using of C++ in C# development environment

All the problems are in "Express".
If you install it, you choose between Visual Basic Express, Visual C++ Express, Visual C# Express, Visual Web Developer Express, Express for Windows Phone. If I'm not much mistaken, you cannot install two or more of these variants of the environment on the same computer at the same time.

This would make development using two language possible but quite inconvenient: either using separate computers or re-installing then back and force all the time :( .

There is one more option: you can install just C++, and C# programming does not need environment, or you can use SharpDevelop.

When you complete your (for example) C++ project, using it in, say, C# environment will not be a problem: you copy just the compiled assembly, in executable form, and later reference it in your C# project. But as to incremental development using two different environments...

...This would be extremely inconvenient. I would not bother, ever.