First off, isn't Mount Hakobe firkin huge? I mean seriously, look:

[img]http://vignette4.wikia.nocookie.net/fairytail/images/f/f3/Mt._Hakobe.jpg/revision/latest?cb=20130705081905&path-prefix=ru[/img]

It's pretty big here. We don't even see the bottom half here I would say. If the mountain is as big as I think, it would not surprise me if two dragons lived on opposite sides. Even if the dragons were to know of each other, it doesn't mean they spoke to each other.

Not only that, while Ice and Water are similar elements, there is enough difference to consider them as different magic. Juvia and Gray's fight could be seen as an example of this.

I can't help on the Acnologia front, but if one first-gen dragon-slayer got through, I don't see how another one can't either.