Debugging through different solutions in Visual Studio

There is no way to have two instances of Visual Studio debugging the same process. This is a limitation of Windows, and most other operating systems in that at most one process can be debugging another.

It is a perfectly supported scenario though to debug binaries that are not a part of your solution. As you've noted you can happily step into binaries from Solution B while debugging from a Solution A.

One item that will get in the way here though is the debugging feature named "Just My Code". This is a feature aimed at minimizing the debugging experience to just the code in your solution. It is great for normal solutions, but bad when you're debugging arbitrary binaries. It's likely causing a lot of the problems around break points you're seeing. You'll want to disable it by doing the following

• Menu Tools → Options → Debugging
• Unchecked "Enable Just My Code"

You can only have one debugger debugging a process at once. So that means you only need one instance of Visual Studio open.

However, you can just open the .cpp/.cs/whatever file from Solution B into Solution A's copy of Visual Studio and set breakpoints. It'll still work even though those files aren't actually part of the solution.