I was trying to install iOS4 (iPhone sdk 4) and also want to have iPhone sdk 3.2. I remembered what I did last year when i want to try iPhone sdk 3.0 beta and continue to submit applications on iPhone sdk 2.2. I created another drive and then install. That work for me but it was not a good solution.

How to setup 2 iPhone SDKs on one Mac is really simple. Yesterday I was trying to install iPhone sdk 4 to my mac but want to continue working with iPhone sdk 3.2. So it was very simple, download the sdk from apple developer site (http://developer.apple.com/iphone) login and then download.

Once its downloaded, simple start the installer and follow what its initial steps but once you reach to "Installation Type" then follow the images below.

Screen shot 2010 06 16 at 7.36.50 PM How to setup/install 2 iPhone SDKs on one Mac
1. Select "Developer" listed below Location and select other. 
Screen shot 2010 06 16 at 7.37.01 PM How to setup/install 2 iPhone SDKs on one Mac
2. Select a directly where you want to install your new iPhone sdk 4. Its a best practice to name the folder with the version number. (Just like in the image) 
Screen shot 2010 06 16 at 7.37.14 PM How to setup/install 2 iPhone SDKs on one Mac
3. Select the new folder you create and then follow the steps to complete installation of the sdk. 
Screen shot 2010 06 16 at 7.37.23 PM How to setup/install 2 iPhone SDKs on one Mac
I hope it might help some one icon smile How to setup/install 2 iPhone SDKs on one Mac