Those are on the small side for butts but even if they were larger, I would put them on the top rack just for ease of cooking. If they are large, I flip them mid cook so there is not one side that is nearer the heat that is on the outside edge of the cooker. With them on one rack (the top), if they are different sizes or one just finishes sooner that the other, it's easier to just take that one off and leave the other one alone. It's obviously easier to check on both of them if you don't have to remove the top grate to get to the bottom one.
For planning purposes, I give my self about 2 hours per pound (usually doesn't take that long) and then put them in a cooler to rest as was mentioned. Not only do I foil them before putting them in the cooler, I wrap them in towels. They stay warm a long time that way (like a few hours easily).