Hello.
That is quite a novel idea! That would sure make things roomier, except for one complication: You need wires to connect the slave drives to the motherboard, along with the Master drive(s), just as if they weren't in seperate boxes. While I'm sure you can extend the IDE/RAID cables to make this possible, note that you'll probably have to keep the two boxes right next to each other, making any bonus from doing this minimal (unless you're really pressed for box space).
It may be simpler to buy a larger box for your equipment, or run a server with the other hard drives in separate towers. But that can run a rather large price tag and takes quite a bit of know-how that I don't know if you possess or not.
