I agree with what most said:
do 2 butts- whats wrong with leftovers? Takes the same amount of charcoal to cook one butt vs two, so make two, and worst case, you have left overs for the week.
I have never mopped or spritz a butt while cooking butts on my egg. I have spritz ribs before, never noticed a difference, so I've stopped doing it. Most people who spritz or mop are using steel non insulated smokers which can lose some humidity. The eggs are good at retaining a humid environment while cooking.
expect 90-120 mins per pound, so with 8 lbs, think 12-16 hours, plus you want to let them rest for 2-3 hours.