copy iPod to iPod
How can songs on one iPod be copied to another iPod?
iPod Touch (6th Gen)
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You sync both iPods to the same iTunes Library or use Apple Music to provide a common iCloud source. This is assuming they are in the Music app.
Turn on [/off] iCloud Music Library with Apple Music - https://support.apple.com/HT204926
"You won’t see an option to turn on iCloud Music Library until you sign up for Apple Music or iTunes Match."