You can't
Your application must target one .NET framework, you can't target several framework.
I would suggest you to target .NET 4 because it's widely installed on windows 7, Windows 8 with 4.5 can run app build for .NET 4.
And upgrading from .NET 3.5 to .NET 4 works fine.
Just be sure to target .NET 4 Client Profile because the full .NET 4 is not installed by default on windows computer.
EDIT: More info on upgrading your code from .NET 3.5 to .NET 4 (I just had to do it a few weeks ago so I know what kind of problem could happend).
It'll be EASY to do it unless your code use server side components (i.e System.Web) which are NOT included in the .NET 4 Client Profile and require the full .NET 4 (not installed by default on end-user win7 PC).
Just switch your project to .NET 4 Client profile and check if it's compile. If it's compile it means it doesn't use any server side component.