I'm more interested in playing bass but everyone I know knows how to play both and they all started with 6 string guitar.
Its more easy for a guitar player to pick up a bass and try to look cool than the vise-versa. But it there is no reason why you can't learn the bass starting from scratch. As a bass player you are a link between the the drums and the guitar so you will have to learn aspects of those instruments eventually. read through www.studybass.com . I like how the author of this site defines the role of a bass player. The bass is as importand as any other instument in a band, so practice up!