how to produce a mix of dll and exe in one C# project in visual studio or other build tools ? Is it technically possible ? Yes I know it can done in 2 or more projects .
I'm not sure what the motivations for your question are, as you can reference a .Net executable as if it were a dll anyway, but you could consider copying the exe file to a dll file with the same name (yes it's a hack but not sure of your motivations).
You can automate this process using a Visual Studio post-build event for your project. This will handle creating the dll copy each time you have a successful build:
copy "$(TargetPath)" "$(TargetDir)$(TargetName).dll"