All early PX10s, PR10s and PA10s have two. Also all of the early Gitanes I'm familiar with. My Japanese Peugeot MTB with Suntour dropouts has two. They make it quick and easy to install the wheel straight.

Some bikes only have one. That means that you have to make sure the tire is evenly spaced beween the stays.

I prefer two, but can live with one.