When you join two contacts, it actually merges the contacts' details in to one single contact, which is very useful for managing your contacts. Separate is and Undo for Join.
So, to join/separate contacts on LG G2 do as follows:
1. On the Contacts application, tap the name of the first contact you want to join
2. Press Menu Button and then select Join.
3. The list of your contacts appears, and you need to select the contact you want to be joined with the already selected contact.
4. Contacts Joined
5. To Separate contacts, open the joined contact, press Menu Button, and then select Separate
6. Confirm the alert message, and Contacts Separated
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