Open-e only works with "physical volumes" or what it sees as physical volumes. If you've made a raid-0 with two drives, you are presenting one Volume that Open-e sees as one physical volume.

The other possibility is that you've not done any RAID setup so far, and you actually installed the OS on the 1st Volume, and in the web interface you are just seeing the second. This is because the Operating System will install and "take over" the first Volume it sees.