Both the fiend totem rage power and the plant domain level 6 power grow thorns/barbs on your body that deal damage to opponents attacking you .
Would the damage from both stack?
Benefit: While raging, the barbarian sprouts dozens of wicked barbs from her body. Anyone striking the barbarian with a melee weapon, an unarmed strike, or a natural weapon takes 1d6 points of piercing damage.
At 6th level, you can cause a host of wooden thorns to burst from your skin as a free action. While bramble armor is in effect, any foe striking you with an unarmed strike or a melee weapon without reach takes 1d6 points of piercing damage + 1 point per two cleric levels you possess. You can use this ability for a number of rounds per day equal to your cleric level. These rounds do not need to be consecutive.
The specifics are a little different because brable armor is more limited in when it applies but deals more damage. So if I rage and activate bramble armor would I deal 2d6+x or just the higher amount?
I would say higher amount. After all, if ad thorns to exixting thorns, it doesn't seem to make that big difference. Obviously it's just my thought, nothing supported by the rules. But the effects seems really similar, so I wouldn't let them stack.