Personally, I use water. As I said, I put pans on the lower grate, so I don't foil the bowl. If you foil the bowl and use water, they say not to foil it over the top of the bowl, or the water can wick out.
Two 8.5# butts? That's going to take a while! Keep them apart from each other...that helps, but it still could take 12 to 16 hours.
You can use the Texas Crutch and foil them when they hit the stall to speed things up, but I like mine better unfoiled throughout the entire smoke. Just my personal preference: I like the crunchy bark, and it's still juicy!
Here's some discussion about foiliing vs. not foiling pork butts:
http://www.smokingmeatforums.com/t/98969/pulled-pork-best-time-to-foil-and-more-importantly-why
If you get done early, you can wrap them in foil and blankets and put them in an ice chest (no ice!). Look at post #9 from the link above...it's explained there.
If you're running behind, you can finish them up in the oven. Some may call this cheating, but the flavor is still good. (I've only done this once, when I grossly underestimated the cooking time.)
Good luck!