Approximate cooking time is based on separate piece weights, and yes, I would figure the 10lb for 20 hours. If you foil when internal temp is around 165-170* it can cut a few hours off the total time. The draw-back is that foiling will kill your bark's texture and soften it enormously, although foiling to rest for several hours can as well.
For placement of the two butts on the grates, I can't say which area will be hotter in the Bradley (middle or top), but if you know which will be hotter, that's where you'd want to put the heavier piece. Do stay well above the water pan...if too close, this area will get reduced smoke reaction and higher humidity which tends to steam the food more than smoke it.
You could smoke at 240-250* instead of the usual 225* you hear everyone smoking at without any noticeable difference in tenderness. Take to around 200* internal temp and poke around inside to feel for resistance. If your probe slides in easy in different directions should be tender enough to pull and rest. You can also just grab the bone with tongs and give it a gentle pull...if it feels like it's ready to pull out, it's time to pull it to rest. I rest my pork shoulder for at least 1 hour before pulling.
Me? I'd get ta smokin' now...the dreaded stalls can add some time...a 2hr/lb estimate allows for some stall time as well, but you need to rest them before pulling, so 20 hours may be jacking things pretty tight on time. If you get lucky and they get done early, just foil and wrap in towels and toss in a warm cooler (be sure to bring one indoors to warm-up if it's in the garage or shed)...butts can sit for several hours foiled/toweled/coolered and still be piping hot for pulling.
Have a GREAT smoke!!!
Eric