dual head HDD

i don't know if there was any ideea like that (i hope it wasn't) 

my ideea is why make RAID(striping) with 2HDD and a chip on the motherboard?... why not RAID(striping) in a HDD? we put 2 heads, a some kind of other chips (RAID like) and that's it... first head is in the inner circle, and secont in the outher (see the picture red and green line) 

the speeds should double... and I don't think that the fabrications cost is double - maybe 130%? 

i guess could be a design with 4 heads too :)