How to

I'm not sure if you mean that both libraries are on the external drive, or only one is, with the other being on an internal drive. And also, are you using Windows or a Mac?

That said, I would think you could just copy the music files from the storage location that is used for one of the libraries, to the storage location that is used for the other one. If using Windows, you would do that with either Windows Explorer or My Computer. The operating system will ask you if you want to replace identically named files. Regardless of whether you answer yes or no you will not wind up with duplicates.

After you've copied the files, I'm not sure offhand if you would need to then use iTunes import function on the files which are new to that location, but you could easily do so if necessary.

Regards,
-- Al

I may be mistaken, but I thought this was a limitation with iTunes. The library sync-ing is one way only and is not shareable with another hard drive, which is in essence what the second instance of iTunes represents. I went through a similar situation a few years back when I upgraded from an eMac to an iMac. There was no copy over procedure for iTunes libraries. I don't know if this has changed any.

Regards,

Rich