When you are installing VS 2008 and VS 2010, will VS 2008 install its own framwork 3.5 or will it use VS 2010's framwork 4? That would be also with VS 2010.

My request is that VS 2008 use framework 3.5 and VS 2010 use framework 4 only.
['https://Stackoverflow.com/questions/5779479', 'https://Stackoverflow.com', 'https://Stackoverflow.com/users/484390/']
When installing Visual Studio 2008, it will install version 3.5 of the .NET Framework. It will not use version 4.0, as installed with VS 2010.

However, both VS 2008 and VS 2010 have multiple-targeting support, meaning that you can choose which version of the framework to target per-project. So, you can create a project in VS 2010 that targets version 3.5 of the framework, rather than 4.0, if you so choose. Obviously you do not have to take advantage of multiple targeting support, if you do not wish to do so. You can retain VS 2008 to develop .NET 3.5 projects and VS 2010 to develop .NET 4.0 projects.

In fact, the default target versions of the framework will be exactly as you expect. VS 2010 will target .NET 4.0, and VS 2008 will target .NET 3.5. You can choose to target earlier versions, but not later versions. VS 2008 cannot be used to develop projects targeting .NET 4.0.
You can select the target framework when creating the project both in visual studio 2008 and 2010 > 

<http://weblogs.asp.net/scottgu/archive/2007/06/20/vs-2008-multi-targeting-support.aspx>