Interesting that there hasn't been much decline in alcohol consumption as marijuana use has skyrocketed -- and the two don't mix well.
Think of it like this: Weed * alcohol. (how intoxicated you feel)
Too much alcohol, you puke, feel like crap, black out, bad stuff. Too much weed, you're a little fried mentally, but fine. The consequence of this is if you drink to a point where you're drunk, but not at your limit, then you smoke weed, it gets you past your limit immediately. Smoke, then drink, though? Now, you get drunker quicker, sure, but you feel it, so you stop before it's too much...assuming you know how to control yourself.
That being said, doing both with reckless abandon is worse than just drinking with reckless abandon. It makes the stupid drunk things you do even dumber.