Now, the basic principle behind sync is to let you have the same content on all devices. However, a small flaw (or perhaps a feature) in this process is where you can't transfer music from one iOS device to another without sweating a little.
Supposing you've got a bunch of music tracks you downloaded onto your iPhone and want them transferred to an iPad, you can't use the copy-paste method that computer users are so fond of. Also note, iTunes doesn't allow you to sync content that way so it's a little hard.
There are, however, several apps that'll let you do this. Sometimes, apps are costly affairs and run either on Mac or on Windows only (sorry Linux fans) but sometimes, things get better and you get free apps.
Things You need:
How to Transfer Music from one iPhone to Another iPhone:
Step 1: Turn off Autosync in iTunes (iTunes Preferences from Menu -> Devices -> check “Prevent iPods, iPhones and iPads from syncing automatically”
Step 2: Connect your iPhone
Step 3: Open iExplorer
Step 4: Click on the device on the sidebar to reveal the sub-menu options “Root” and “Media”
Step 5: Click on “Root” – this will reveal a list of folders on the iPhone
Step 6: Click and expand “Media” and then “iTunes_Control”
Step 7: You'll see the “Music” folder under this. Drag the folder to desktop or any other folder to extract all the contents
Step 8: Disconnect your iPhone
Step 9: Open iTunes and drag the Music folder into a new library (create a playlist for easier access)
Step 10: Connect your other iPhone/iDevice.
Step 11: Select all the recently copied music and drag it to the device's name on the sidebar of iTunes. This will sync all the music to the iPhone/iDevice once you press “Sync” at the bottom.
So that's how you sync from one iPhone to another. Not exactly a cakewalk but you'll get used it if you do it a few times.