You can sync multiple iOS devices to one computer and one iTunes account. In fact there are a few ways you can do it.
Method One
If you share your computer with other people, you can maintain separate user accounts for each person who uses the computer. This is useful especially if each user has different tastes in music or other media; users can maintain separate, personalized iTunes libraries and can customize their devices' sync settings accordingly.
Windows users:
Look in your computer's Help documentation to learn how to set up additional user accounts. To locate this information:
1.Click the Start Menu, and then click Help or Help and Support.
2.Enter "new user" in the Search field.
3.Press Return.
Method Two
You may use multiple devices on a computer without the need to create multiple user accounts. To do this, you can set iTunes to update each device with only certain playlists. This method allows you to put all your music on, say, your iPod and keep your iPod shuffle updated with only your workout music. Here's how to do this with iTunes:
1.Create new iTunes playlists that contain all the music that you want to sync to each device.
2.Learn how to sync your device.
Method Three
Create a separate iTunes library for each device. Note: It is important that you make a new iTunes Library file. Do not just make a copy of your existing iTunes Library file. If iTunes is open, quit it.
1.Mac users: Hold down the Option key while you open iTunes.
Windows users: Hold down the Shift key while you open iTunes.
2.In the dialog that appears, click Create Library.
3.Name and save the alternate library file. iTunes opens with your brand new library file.
4.Open iTunes Preferences. Choose iTunes > Preferences (Mac) or Edit > Preferences (Windows).
5.Click the Advanced tab.
6.Deselect the "Copy files to iTunes Media folder when adding to library" checkbox if it is already selected and click OK. This will prevent iTunes from creating duplicates of your existing media files in the new library folder.
7.Add the specific media files you want on your device to iTunes either by dragging them into iTunes or choosing File > Add to Library (Mac) or File > Add File to Library or Add Folder to Library (Windows).
8.Once you have the iTunes Library just how you want it, connect and sync your device.
9.Once the sync is complete, quit iTunes.
10. Mac users: Hold down the Option key while you open iTunes.
Windows users: Hold down the Shift key while you open iTunes.
11.Click Choose library.
12.Choose your original iTunes Library and click Choose (Mac Users) or Open (Windows Users). You can now switch back and forth between the different libraries for different devices.
Source(s):
More information here on How to use multiple iPods, iPads, or iPhones with one computer:
http://support.apple.com/kb/ht1495