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Let's assume User 1 is the primary user account and iTunes library (associated to the Apple ID) that you want to use from this point on. If you have a copy of your music folder from your second user account saved to an external drive .... you can open your iTunes library on the User 1 account and Add the music to that library. (See link below). Then you can check for duplicates and eliminate them.
If you have a lot of iTunes purchases on the two user accounts, another way to approach this is to combine those purchases. The information below is from the "Frequently Asked Questions" section of this article:
I have purchased music, apps, or books with multiple Apple IDs. How can I get all of this content onto my iOS device?
First, you need to copy all of your purchased content so it is on the same Mac or PC with iTunes. This computer should be the one you sync your device with. For more information on how to move your content, see these articles:
Next, authorize your computer to play content with each Apple ID in iTunes. Once your computer is authorized for all your content, it can be synced to your iPhone, iPad, or iPod touch.
I hope this information helps ....