You can use LVM, and include both the disks in one volume group. See http://www.tldp.org/HOWTO/LVM-HOWTO/index.html.
You stripe the logical volume (see http://www.tldp.org/HOWTO/LVM-HOWTO/createlv.html
), I prefer a stripe size of 32k over the 4k in the example.
Alternatively, you can use software RAID level 1, where there are also striping options, but I prefer the flexibility of LVM myself, and save using RAID devices for level 5 where I can have redundancy.