With RAID the RAID drivers control where the files are sored and how.
RAID 0 - You have one drive with the total capacity of twice the smaller of the two drives. ie 20 Gb + 40 Gb drive = 40Gb drive, 80Gb + 80Gb = 160 Gb drive. The two drive act as one drive. If one drive fails you loose everything. Hence RAID 0 isn't really a RAID at all. RAID = Redundant Array of Inexpensive Drives. In RAID 0 there is no redundant drive.
RAID 1 is slower on writes as everything needs to be written twice but is faster on Reads as it can Read simultaneously from both drives. But you have the redundancy that means if one drive fails you still have a copy. You still only see one drive.