Partitioning 2 volumes

Hello guys,
I need help here, all the others topics are too old and don't fit with what I'm trying to achieve.

I have a 2To and a 4To loaded in my ReadyNAS Duo, and I'd like to separate in half the 4To into a 2To Raided with the first one, and the other 2To.
The raid will backup my pro stuff, the other one will save all of my music and my films.

I've already backed-up my volumes, so they are fully ready to be formated.
I tried some stuff, but it doesn't work so I'm kind of lost.

Any help is appreciated !
Thanks