Several of my colleagues have multiple passports, simply because they travel often. They can travel with one passport, while the other is sent to the embassy for the next trip's visa.

Two of them are British, as am I. I looked into the process, but then changed the focus of my work and didn't need to travel so intensely. Essentially, I would have sent in a normal passport renewal form, and attached a letter saying "I'm going to X countries in the last Y months, so need a second passport". "I'm going to X as well as Y" is an equally valid reason.