one hand good vs single enemy, like boss
two hand good wiping out mobs

Well, if you wan, you can master both trees.

Yes u can, if you don't mind the hard works.

Keep your level as low as possible until you possess 2 skill rings (forgot the name, +x% sp), then play it normally or farm if you wan, every time just before a character levels up, swap the 2 skill rings in, after level up, swap back the normal accessories, and repeat for each character, keep checking their experience level so you don't miss.

I did this too, but missed 20-30% of the levels, at level 60+, I've completed one tree and half of the other.