I got this off of Northwest Kite Forum. It is very useful information and is something alot of Kiteboarders take for granted. It is a major safety concern so please read. The doubleblade knife is available in my store. It is carbon Fiber (light) with easy to hold shape.
Nak wrote:
I hurt my back yesterday, so I'm off the water for a few days. Got bored, so I ran some tests with my force gauge. Pretty damn eye opening. I used two knifes: a standard single bladed hook knife that comes with most harnesses, and a "Captain Hook" double bladed hook knife. Tests were done with a force gauge on a test bed. Q-line and 800 pound spectra were cut.
Here's the results:
SINGLE BLADE HOOK KNIFE
Q-LINE FORCE REQUIRED - 58 POUNDS
800 POUND SPECTRA FORCE REQUIRED - 184 POUNDS!!!!! : 
DOUBLE BLADE HOOK KNIFE
Q-LINE FORCE REQUIRED - 2 POUNDS
800 POUND SPECTRA FORCE REQUIRED - 2.5 POUNDS
Needless to say, I'm ordering a couple of more Captain Hook knifes, and immediately throwing out all of my single blade hook knifes! I don't want to get into a brand thing here, and there are other dual blade hook knifes. But, there ain't no damn way you're going to cut 800 pound lines, in the water, in an emergency, with a single blade hook knife! I thought this was pretty important to pass on right away.I tested a third knife, also a double blade hook knife. This one comes standard with a certain brand of harness... Anyway, 7 pounds to cut Q-line and 11.5 pounds to cut 800 pound spectra. Very acceptable.
The only tension on the lines was that applied by the knife. In the case of the single bladed knife, that was a LOT of tension! 
I wanted to test worst case scenario, and the lines you have to cut may not have tension on them.
I just tried rigging the lines under tension, but due to the geometry of my test bench I was unable to actually measure the results. (I could set it up so, but it would be more work than I really want to do. ) Results seemed to be similar. Getting the Captain Hook knife anywhere near, like within 6 inches, a kite line under tension resulted in a cut line. 
Seriously, it went through the line like it wasn't there. I was unable to cut the 800 pound spectra with the single bladed knife with this setup. The blade was damaged by the previous cut, but that is real world too, I think. If you have to cut one line, you'll probably have to cut more. Also, I was holding the knife instead of having it rigged to a test bench. It's damn hard to apply that much force with the little handle...
The negative to the double bladed hook knifes is that the size of what you can cut is limited.