|You can have multiple raids on one controller channel. Depending on controller you can have multiple raids across multiple channels. |
This means if you have one channel that has 5 slots you could have a mirror set and a raid5 set on that channel. Multiple channels give you more flexibility and will allow you to raid across the two channels.
Right now I would suspect [though opening the case and confirming the cabling would go a long way in knowing if those two slots ard cabled to the raid controller] you have one mirror set with an available slot and one raid 5 with one available slot.
Those slots should be occupied by hot spares.
Otherwise you should be able to drop identical drives into the two slots, go into the perc bios or via desktop software and configure the two drives as a mirror set. Then you reboot, let the array build and then go into disk management and partition/format the volume.