There may be some confusion here.
Devices sync with a single library, some may be manually managed from multiple libraries, but an iPhone can only be synced or manually managed from one library. Switching to another removes the existing media.
HT1535 - Managing content manually on iPhone, iPad, and iPod
Up to 5 computers can be simultaneously authorized for each Apple ID. iPods & iOS devices are limited to DRM protected content from up to 5 different accounts. There is no limit to the number of devices that you can load up with content using iTunes. You can deauthorize computers that you no longer wish to use to access protected content so that you can authorize others. You either do this on the specific machine or deatuthorize all and then reauthorize as needed.
HT1420 - iTunes Store: About authorization and deauthorization
A mix of up to 10 computers and iOS devices can be associated with your Apple ID for iTunes Match or automatic downloads of past purchases. There is a 90 limit before a computer/device can be switched to another ID.
HT4627 - iTunes Store: Associating a device or computer to your Apple ID
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