WTF. How is this at all balanced?
Ok, I'll bite. 5/5 for 6 isn't exactly top tier stats, Bolderfist is 6/7. There are quite a few 1 Atk minions at 3 cost, but the majority are 2 or 3 (and a few 0s in there too). Health values also vary wildly between 1-4. So you can reasonably expect to get at least 7/7 stats from this split across 2 bodies.
It's a very strong card, in a good number of cases better than Boulderfist. However that is it. It is a totally vanilla minion otherwise. You can't really count on getting any specific effect from the 3 mana summoned minion, but there are lots of interesting ones: Flamewaker, Alarm-o-bot, Acolyte of Pain, Divine Shields, Charges, taunts, etc.
This card is really good, but you basically get a roll of the dice for a 3 drop and a 5/5 neither of which you can assume will have any specific effect. In arena this card is beyond amazing, I'm assuming this is a TOP PICK there. In constructed, depends. I can see a world where this doesn't do anything specific enough to be worth it.