I think darniil was trying to make two points. The first is that though Ruin and Preservation have not merged together, they are linked in a fashion. Let's use legos as an analogy.
Let's say we got two legos, a blue one and a red one, named Preservation and Ruin respectively. Some guy named Sazed goes over to the the legos and picks them up. He holds the blue lego in left hand and the red lego in his right hand. Thus, the legos are completely separate from each other, and the only connection they have to each other is that they are both being held by the same person. This is the way most people seem to view Sazed's current state in relation to the Shards he holds.
Not let's consider another example. Sazed goes over and picks up the blue and red legos and smashes them together somehow. Poof! Now he's just holding a single lego, and it's got a new color, purple. This is the idea that Chaos and KChan so vehemently disagree with, the idea that the Shards have merged and now have a new intent. This is also not what darniil was suggesting.
Final example. Sazed goes over to the two legos, and sticks the red one onto the blue one. They are now held together in the standard lego way and can be moved about as though they were a single piece. But they are still separate. The red lego is still red, the blue lego is still blue, they are still two different legos. But they now have a connection to each other, a connection that is more than just "they are being held by the same person" like in the first example. This, I believe, is what darnill was thinking.
Basically, his idea is that when two Shards come together, there are more than just two possibilities. It's not just "they become a single Shard" or "they remain totally separate with absolutely no connection to each other." It's possible for them to become linked together while remaining distinct, or so darnill is speculating.
The second point I got from darniil's post was that just because Ruin and Preservation remain separate Shards now (or so most of us think) doesn't mean they will remain that way always. Perhaps, if Sazed holds them long enough, they will indeed combine into one. The red and blue lego attached to each other will become a single purple lego.
As for my own views, assuming the above understanding of darniil's points is correct, I'm not sure. I do think that there is a way to merge Shards together, but I don't think it's something simple or something that will happen automatically just by Sazed holding two Shards together. I'm also not sure whether I think it's possible to merge only some of the Shards together or whether you have to merge all 16 together at once. Regardless, I don't think Ruin and Preservation will ever become a single Shard just by Sazed holding the both of them for a long period of time.
As for whether Sazed is holding two legos separately or has attached them to each other, I also am unsure. It's something to think about I suppose.