i've seen people play those big wooden zylophone thingies (yes i forgot what they're called) with two sticks in one hand, how exactly do you grip the sticks so it's not really messy? or is this just a very messy and sloppy technique and that's why i hardly see anyone do it

There are a couple of different ways to hold two mallets (or sticks) in one hand.  The one that Random describes sounds like the Burton grip, which is one of the more intuitive four-mallet grips (two hands, two sticks each, total of four).

Start by holding a single drum stick using "matched grip."  Slide a second drum stick between your index and middle fingers, with this second stick resting flat against your palm (between the first stick and your palm).  You can change the interval between the ends of the sticks by moving the inner mallet (the one your thumb is in contact with).  For larger intervals, sometimes your thumb will rest over the inner stick.  (Tough to describe without pictures - but imagine the basic motion of a bass player "slapping" a note with his thumb - it's kinda like that, but not exactly.)

Holding two sticks in one hand can get a bit cumbersome with drum sticks, due to the diameter of the sticks.  Maybe go with a thinner stick, or hold onto the tip end in your hands, where the stick is a little thinner, and strike with the butt ends of the sticks.

Just wanted to add something.  I dont have much exp with mallet playing, but I do have a lil bit.  Not playing with 4 mallets worth, but I have seen guys play with 4.

Anyhow, by changing the angle your are playing the keys from(with your arms) you can create less space between the 2 in one hand.  Kina of hard to explain, but Ill try this way:

Imagine the keyboard with a clock face lying on top, 12 at top 6 bottom.   Your right arm naturally will approach the clock from a 4-5 ish(on the clock) direction.  The more you come from a 6 direction, youe more you will be able to spread the mallets for long stradles.  Conversely, the more you come in from a 3 oclock ish angle, the more you will be able to squeeze the 2 mallets in the one hand to play even on the same key at the same time.  

Is that a valid technique, James?

There's another four-mallet grip that Gordon Stout uses, which is similar to the Burton, but has the second mallet held between the ring and middle fingers.  This makes it tougher to close the interval between the mallets enough to play a major second on a marimba (i.e., play a C and the D next to it), so one way Gordon suggests playing that spacing is to do basically what you've described.  With the Burton grip, tho, it's possible to have the heads of the mallets (or, the ends of the drum sticks) virtually touching each other, so it isn't as necessary to change the angle of the forearm to accomplish this.

And there's the freakish right hand grip of Mike Mainieri who holds the outside mallet (right hand) with only his pinky finger! On drums ... THAT'S gonna hurt!

When applying 4-mallet techniques to the drumkit, I prefer the Traditional or Burton grip for power playing ... Musser or Stevens grip for moderate to soft playing. I use the latter a lot when I need to play suspended cymbal rolls with one hand (2-mallets).

There's also the 'bundle of sticks' approach where you just grab the sticks so they are against each other. Steve Gadd did this with his quasi-Mozambique groove on Paul Simon's "Late In The Evening". Makes it sound like you've got more than one guy on the drums.

This is what I do.   The spacing may be odd on keyed instruments, but it seems to work well on kit, as drums are often larger than keys, you might need a bigger reach between sticks.  

I've seen Jim Keltner perform several times and he will use combinations of sticks and other percussive items (maracas, shakers, etc.) along with sticks.  Makes for a very full sound.