How often do moons eclipse one another?

In my fantasy book series, the world I write about has got two moons. I thought it would be fun to build a festival around a idea where once a year one of the moons eclipses the other. Scientifically, how far-fetched is this idea? For instance on Mars, do Phobos and Deimos ever eclipse one another and how often does it happen? I'm not opposed to stretching the truth just a bit since it is a fantasy book, but if it's completely scientifically ludicrous or impossible, I'd rather not include it. Thanks for the help