Here's an easy way to switch from one user account to another quickly in OS X. 1. Open System Preferences. 2. Go to Users & Groups. 3. Click Login Options at the bottom of the user menu on the left. 4. Be sure that you check the box next to "Show Fast User Switching Menu As." This will let you choose other user accounts from the menu bar. You can pick the one you want, type in your password and you'll be switched to that account while still being logged into your first account.
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Skirting auto-login in Lion | Macworld