I'm sure the room is there because it's the same box as the Mini Server box, which has 2 drives. You'd have to get the cable from someone with an old server box or wait until someone gets clever enough to make a cable for a second drive, I'd imagine.
As for me, I'm buying the server b/c I need the quad core and having 2 drives is the icing on the cake... I'll just have to be patient to wait for the Thunderbolt external GPU to do some serious gaming after-hours.