The reason I like two jacks is for convienience. On your daily driver, you can rotate your tires in a fraction of time with two jacks. It is also easier to put both sides of the car on jack stands if you can pick up the whole front or rear at the same time. Some cars can be lifted by one jack underneath the center of the car, and others need to be lifted from the outer edge (most unit-bodies). It is just a matter of preference. Also, one day you are going to blow a seal in your jack and if you don't have a spare at that moment, you will be hating life.
For what they do, jacks are cheap... buy two!
