I have the same contacts but with different details, for ex: I have contact 1-A which has the phone number and an email and contact 2-A which has the birth date and other phone numbers. When I select one of them and tap "Join" they successfully join.
Since I have two ROMs that I can boot between I want the contacts to be the same on both of them. When I go in contacts in the other ROM which is CM12.1 it tells me to connect to Google, that's what I do. Then when I check the contacts they are separated again.
I have an OnePlus 2 running stock OxygenOS version 2.1.1 on top of Android 5.1.1 which is my daily driver and CyanogenMod 12.1 also on top of Android 5.1.1