To do handseals themselves take lots of practice and understanding. What I think Haku did through all of that training, is take the concept of performing jutsu with 2 hands, apply it to his training, took that concept to the next lv and with the guidence of zabuza, who likely gave him that concept, created a method of training, that only Haku could accomplish with his talents and surpass Zabuza with that(along with many other things).
question... did rasengan need a hand seal?
I am going to go out on a limb here and say that 1 handed seals are probably not a good thing.
Going to spoiler this just in case anyone doesn't want part of the story for fairy tail told
My point here is that jutsu created with 1 hand may not be as powerful as that created with 2 hands, so the reason we don't see it often is simply because, ninjas only use them in situations where they can not use both hands for 1 seal.
deidara was probably making clay bomb in 1 hand when he was going to detonate the bombs with 1 handed seal.
Zabuza had kakashi in a water prison which was much more important to him than killing the genin brats. so he used 1 handed seal to make the clone
Even in the case of haku, she/he needed to use 1 handed seals to combine the 2 elements, its not like she had 4 hands to make 2 sets of seals.
did nothing i said make sence enough to seem as a possible reason why 1 seals arent used as often as the 2 seals? likes 3 pages ago.... >.>
I totally spaced on this before... I knew that the hand seals were Mudra, but didn't even consider that there are one-handed Mudra as well, there are more one-handed than joint ones... so I'd say now that he was just more versed in Mudra, and their meanings, so he could use all of them not just the joint ones. I guess that could make Haku a Buddhist priest, could be a connection with not wanting to kill. oh well