How do I maintain two different iOS devices on a single iMac
Easy way: create multiple user accounts on your Mac, a tutorial can be found here.

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Hard way: create different iTunes libraries for your user account.  You'll need to:


Quit iTunes
Hold down the Option key and then launch iTunes
Click the "Create Library..." button on the dialog that is displayed
Once you've created the different libraries, you need to remember to use the Option key and then "Choose Library..." to switch between the two every time you start iTunes


A multi-part tutorial can be found here.