install iphone sdks side by side

Question

Im trying to get my xCode to contain all iPhone sdk's from 4 back to 2. But when I download the DMG files, it seems to only allow me to have an xCode with SDK versions 2 - 3.1 OR 3.2 - 4, not all together.

How can I install the SDK's into one xCode so I can build for all different OS versions?

Thanks

EDIT::

Ok so I know I can set the base SDK in xcode, but the options I have are only 3.2 or 4.0, I cant seem to install the 3.1 or earlier SDK's how can this be done?

Final Edit::

Ok got it solved, basically you only need to have an older version of xcode installed to get earlier simulators running, otherwise the articles given to me in my answers as well as others were very helpful:

http://www.clarkcox.com/blog/2009/06/23/sdks-and-deployment-targets/

Install xCode 3.2.3 w/ iPhone SDK 4, get "Base SDK missing", can't see other SDKs

How To Make iPhone App compatible with multiple SDK (firmware) versions

http://cocoawithlove.com/2010/07/tips-tricks-for-conditional-ios3-ios32.html (possibly the best one)

Answer 1

Is there any particular reason you need to do this? Are you trying to develop applications that will run on iOS 4.x and older versions (3.x)? You can still develop applications which will run on iOS 3.1.x with the iOS 4.x sdk.

Take a look at the following article: http://www.clarkcox.com/blog/2009/06/23/sdks-and-deployment-targets/