When I was in college I worked two jobs for a while. Once in a while I'll see people encourage others to get a second job to make more money. A friend of my wife's had been working two jobs until recently. I've done it myself in the past briefly and I know others do it but I don't know how common it really is. How many people actually hold multiple jobs? Easily answered... BLS has data on multiple job holders.
In 2010 there were about 6.8 million people or 4.9% of the population working two jobs.
Hmmm... at the same time about 10% of the population was unemployed. Those dual job workers are hogging jobs! I'm mostly kidding here. I'm sure most people working multiple jobs are not doing so for fun and they likely have to work multiple jobs to support themselves and their families.
A little over half of the people with two jobs have a full time job and a part time job. About a quarter have two part time jobs. A small amount of people actually work two full time jobs (80 hours a week).
Here's a look at what Part time (PT) versus Full time (FT) jobs are held by the people working 2 jobs :