I built my enclosure out of 11 ply birch, and it weighs just under 45 lbs. I didn't want to use MDF because of the added weight. The driver itself weighs closer to 45lbs, so total weight of completed sub is 90lbs. It will shake in it's place pretty good, but it's only noticeable if I'm really close to it. (I could put a magazine on top and it would shake it off, but it wouldn't shake a hardcover textbook off)
I think your 12" sub idea may be futile. You may notice some added output between 30-80Hz running the 12" with the 18", but you'll be limited to the 18" for anything below 20Hz. The response in your room may seem alot more uneven this way, and may give the illusion that you are lacking in the ULF department.
Also, I'm not sure what spl levels you are currently used to, but the single 18" with 900w should be more than enough for you. (If you're used to commercial or ID subs up to a point.)