I think you should be able to, but it will depend on your implementation. I know pure software raid in linux can do this. For hardware and driver-level raid, it depends on your system, so you'd probably just need to google it for your particular hardware controller. In any case your performance would be limited by the slower of the two disks. This is one reason why matched HDD's are preferred.
That brings me to the second part of your question. RAID 0 is also referred to striping. Simply put, with RAID 0, your data is stored alternating between the disks. Part 1 of a file is on disk A, part 2 on disk B, part 3 on disk A, part 4 on disk B, etc... When you read the file, both disks can run at the same time, so you can get part 1...