Only thruth I know is that there's a repository: H'uru Team's where the TOC team goes to grab stuff to include in their shard. There's also a different timeframe. TOC MOUL is here since nearly a whole year where Gehn is rather a new born. The TOC team also develops things on their own. So with all of the above said, I would tend to say this:
TOC & Gehn are very different shards. Since the TOC team imported the physX fixes made by the H'uru team in their shard, they imported the bugs that came along with. Other than that, I think TOC shard is very different from Gehn shard.
The biggest difference between the 2 shards as of today: you can complete all the game (every journeys + features (sparklies, etc.)) in TOC which isn't true at all in Gehn.
Another example: TOC has the Wall and H'uru team is working on it for Gehn. They are not even using the same exact code.
EDIT:
It was Mystler's post you were responding to not mine 
... anyway I think my theory has some thruth even though Mystler is saying they are BASICALLY the same. In fact, BASICALLY doesn't equal EXACTLY, so the "basically" would be the above-said comments 
