easy question !!!

I have 2 17GB disks
I made a logical partition on both of them for the full 17GB and formatted them as NTFS
I close the disk administration go back into it and try and make a mirror of the two disks.
I follow the recommended method (click on one partition, press cntrl and choose the other patition)
It shows in the status bar at the bottom multiple selection so it sees both drives.
I then click on fault tolerance but I dont see the mirror option hilighted !!
Why cant I mirror these two drives ?? they are exactly the same !!