Certain people can have two passports. If you are a diplomat you can have a regular and diplomatic passport.
The general rule is that someone cannot posess two U.S. passports at the same time. There are exceptions but for the regular traveler they do not apply. I don't think us "cruisers" would qualify. BTW: If a second passport is issued it is only good for two years.
Here is the State Department link that explains things as clearly as the government can.