Is two toenails uncommon? Should I be concerned?

One reason for one toenail growing on top of another is, as Alison suggested, a trauma to the toenail.

For example, my first toes are longer than the big toes. As a distance runner, even in properly fitted shoes, those two longer toes hit the inside of the toe box when I'm running downhill. The effect is known as "black toenails," the blackening caused by bloody blisters under the nails. The blisters will eventually heal, but the nail bed has been traumatized and the damaged nail will eventually drop off as the new nail grows underneath it.

Your podiatrist can help you determine the cause of your double growth. If it's caused by some repeated trauma to the toes, you may have to have your shoes fitted better.

Just one possibility.

I went to a chiropodist who told me that my toenail discoloration was because the nail had become detached. When she clipped it shorter, she noticed a similar thing, that I actually had one toenail growing on top of the other. I still don't see it myself, but that's what she said. Her advice was just to leave it and see what happens, given that any further action she would take had some chance of losing the nail entirely (maybe not a bad thing, in my opinion). I haven't yet sought an opinion from a podiatrist, but that will be my next step. I'm in Ontario Canada, so your definition of chiropodist versus podiatrist my differ.

I would suggest seeing a podiatrist (foot doctor), as you may not have any trouble with your double toenail now, but it would be wise to receive information about your specific condition and the underlying cause, in order to prevent any infections from developing. I assume it may be difficult to keep clean and to cut/trim your toenails?

Did you have a trauma or injury to your toenail?

You can locate a podiatrist by state at the link below:
http://www.epodiatry.com/usa_clinics.htm