AG concentrate?

Is there any problem with doing a concentrated wort boil and then topping off with sterilized cool water when transferring to primary? I have an 8 gallon kettle but would like to be able to do a concentrated boil like I used to with extract, then top off with water in two separate carboys so that I can pitch different yeasts, dry-hop one of them, other random additions, etc. Are there any issues with doing this?

The net result of doing concentrated all grain is loss of efficiency. Efficiency already goes down as your OG goes up, but diluting with water makes it worse. You're trying to get 10 gallons finished out of an 8 gallon pot? I'd hold on to the 8 gallon pot for heating water and upgrade to a 15 gallon boil kettle.

Yeah, at a certain point you can only extract so much from a mash, so you're going to be running up against a wall. Let's say you want to make a 1.05 10-gal batch. To get that, you'll have to wind up with a 1.1 5-gal batch, which is pretty hard to get, and you tend to be lucky if you get about 65% efficiency with a beer that big. Bobby's plan is probably the best. If you don't want to buy a new kettle, though, you could always treat your batch like a 10-gal partial mash. Mash a batch at a point where you've had a decent efficiency in the past (say 1.07 final gravity), then do a late extract addition to make the rest of it up. At that point you could dilute it just like people doing a partial boil/partial mash batch.