If the Legendary is obtained roughly at the same time, why wouldn't good 25 man guilds split their raids and obtain 2 Legendary weapons at roughly the same time? If it's possible to get them at the rate of Shadowmourne, you could outfit your raid with them fairly quickly by doing this.
However it's going to be done, I wouldn't be surprised to see some top guilds split if it is that much of an advantage. I'd also hate to see it be something that's so difficult to get that it's like Atiesh. I mean, you could easily get 4-5 shadowmournes.
If it's very slow, why wouldn't people split to two 10 mans? I'd bet a lot of guilds would if it was an advantage in progression. Do you know how many times I've seen people win warglaives to jump to high end raiding guilds who end up having 4 people with double glaives?
One thing is for sure. Two 10 man guilds can get two legendary weapons at roughly the same time. If they then decided to form a 25 man guild, they may have an advantage. In the meantime, our 25 man guild can only get 1 per 25 people while a 10 man gets 1 per 10 people.