Two people huddling together would survive longer because the cube-square law takes effect. Two people have twice as much mass as one, and twice as much heat generating capacity, but they don’t have twice as much surface area exposed to the water. Whether it would make a significant difference, I don’t know.
No, a wool overcoat won’t behave like a wetsuit at all. “trap” is the operative word. Wetsuits are skin tight, so the water that’s next to your skin stays there. An overcoat isn’t anything like skin tight, so there would be plenty of circulation of cold water around your body.
A wool overcoat won’t work as well as a wetsuit, but it’ll still work some. Wool is well known among natural fibers for retaining most of its insulative value even when wet, which is why it was so popular among sailors. I’ve heard that wolverine fur is even better, but wolverines are a lot harder to raise in captivity than sheep, so it doesn’t get used much.
Keeping you warm while wet above the water means nothing if you’re actually immersed in water. If it allows cold water to circulate against your skin, it’s useless, regardless of how good an insulator it is.
Its behavior above the water matters if you’re only partly immersed, as most of the survivors were. And even under the water, it’ll slow the circulation of cold water, even if it doesn’t completely stop it (heck, a modern wetsuit doesn’t completely stop it, either).
Yeah. I wasn’t trying to suggest that it was as good as a wetsuit - just that it might be better than nothing.