I've tried it once or twice, but only when I had a VERY LARGE backstop and there was no danger of rounds overflying it, posing a danger to anyone beyond the range.
Let me tell you a story about learning the hard way:
A friend and I went to the gunshow several years ago and picked up some 30 round mags for our Ruger 9mm's (my P89DC, his P94). We went to the local gravel pit/firing line with another friend to try out the two-handed, two-gunned, Hollywood stunt. The other friend asked if he could try it first and we relented. BIG mistake. He fired them as fast as he could, with little or no regard for keeping the guns aimed at the backstop. While we were reloading to try it ourselves, a guy pulls up on a 4-wheeler and tells us that rounds were striking the trees near his mobile home down the road from the pit. We apologized, admonished our buddy, packed up, and went home for the day.
Lesson learned: If you're going to try it, be cognizant of recoil and of keeping the muzzles pointed at the target before pulling the trigger! Don't try to turn them into FA bullet hoses.
Frank