> what happens to the condensation that builds up under the fly if you cannot
> separate the fly from the inner.
Well, in many years of snow camping with a double-wall tunnel tent (which I never split up), that has never been a problem. If the condensation is water, it seems to evaporate and dry off fairly easily. Anyhow, the light inner fabric cannot hold much water, so there's little to worry about. And pitching the inner first in pouring rain is simply stupid imho.
And if the condensation is ice ... well, tough. I do remember once opening my tent up 2 days later after we had got home, and the ice fell out of it. Fortunately, I did that outside! Normally, the inner tent sheds the ice next evening.
It really is NOT a problem, ever.