You're thinking about it the wrong way around. You don't create partitions and then mirror them, you create the mirror and then partition it.
So if you have two 1TB drives and you want to use mirroring, you create a mirrored RAID volume from the two drives giving you 1TB of usable space. When you then divide that up into two 500GB partitions (let's call them "D:" and "E:"), you end up with a copy of the "D:" partition on half of the 1st 1TB hard drive and a second copy on half of the 2nd 1TB hard drive. The same applies to the "E:" partition.
You can't start with two non-RAID drives, create a partition on it, and then mirror the partition (well, OK if you're using Windows software mirroring you can, but few people do that these days, especially for servers where considerations like being able to hot swap a drive are important).