A lot of the traditional big beers were based upon long boils, up to 4, 6, even 8 hours (probably even more). You will get more caramelization, and definitely go through some propane, but not enough that it should discourage you from doing so. You'll want to make sure when you sparge to keep the pH in a good range so you don't extract tannins, since the tannins will also concentrate in the final product. Two pots is a good idea if you can handle it. The greater total surface area of boiling wort you've got, the more you'll boil off. I'm not sure what the effect of boiling hops for 2-2.5hrs would be (other than a very small increase in bitterness), but you could always wait until you think you've got about an hour or hour and a half left, then toss the hops in.