No don't do it. It could even damage your system by creating a "loop".
If you want a faster drive, you need an enclosure like this one at OWC and configure it for striping (RAID 0) per notjustjay's comment above.
Honestly, if you need RAID striping, you need to use eSATA. Firewire 800 has a limit of about 100MB/s, and SATA can go up to 150 or 300 MB/s. The issue is if your drives will be fast enough to saturate those interfaces. Take those into account. Do a bit more research.
Short answer: You won't be able to double your Iomega drive's speed unless you get a new drive that does RAID striping over SATA.