I've already tried this & failed miserably, years ago I thought about doing the same thing, wasn't till late last year I finally got around to it... (After I set up a software RAID 0 setup, I attempted with 2 USB Flash drives, had already installed Quake 3 Arena on one drive, ran pretty fast, but wondered if possible to run 2 USB 2.0 Flash drives in RAID 0 & see how much improvement I got, but didn't work too well)
though I tried with 2 USB Flash drives, neither would show up in Disk Management.
so not sure if 2 regular HDD's will show up using a USB connection.
if it were 2 eSATA drives, that might work.
but as to 2 USB drives, not sure.