It sounds like you want to sync your iPod with music on two different computers. To be able to sync your iPod with multiple computers, you will need to put your iPod in manual mode:
By default, your device is set to "Automatically sync songs…," sometimes called automatic mode. To transfer music from multiple computers, your device must be set to "Manually manage music," sometimes referred to as manual mode.
You can find all the steps to do this and more information at the full article:
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