I'm consdering brewing a simple all grain recipe for my first, most likely a Hefeweizen or Belgian Wit with approx 10#s of grain. My experience is approx. 15 brews using Partial Mash and/or Extract + steeping specialty grains.
No. You really can't top off like that will all grain. Unlike extract where you have all the sugars condensed in your pot, if you don't collect the sugars from the tun then all you're doing by adding water post boil is lowering your gravity.
If you have 2 pots you can split your boil in 1/2. 3.25 in each pot and split your hops for both.
I had the same question and this is what I'm going to do. The only tricky part will be hoping the boil starts at the same time or else I will need two timers. I have two five gallon coolers and am also considering splitting grains for high gravity beers. So two mash tuns and two pots would make sense. My electric stove would tolerate this better than a giant 8 gallon pot!