I've got a handle on how to install Xcode 3 and 4 on the same machine (running OS X < 10.7), so I'm looking more how to manage this environment.
Rather: is it possible to enter some shell command to "point" to the Xcode version you would like to use for a certain task?
For example, some packages in Homebrew seem to have issues with Xcode 4 compilers (multiple
-arch flags not supported any longer etc.). So, when I want to
brew install something, I would like to use the old Xcode 3 gcc suite. But when doing actual Cocoa development, I would rather like to use Xcode 4.
RVM also does a nice job of managing work in multiple environments. I think, newer Linux distributions offer a tool called update-alternatives to achieve such a goal. Just to give you more of an idea of what I am talking about.
Similar to running both simultaneously except I'm looking to use them sequentially.