Not very hard, but to hit somewhere effective is harder with two blades... Unless you are very skilled, to hit somebody however is another case, thats going to be easier. But a light touch on the leg isn't going to anything but hurt the opponent. With one lightsaber you can cut the entire leg off...
Thats the difference, Anakin wasn't skilled enough for one blade against Dooku however. he had a better chance of hitting Dooku with two hoping it would wound him so he could finish the job...