I play bass and guitar so I don't really see any reason why you shouldn't. I'm better at guitar but I still feel a need to keep up with the bass (probably because I write and record all my stuff myself and if I can't keep up with what I write then I need to practice more). But as stated above by Ferguson, bass and guitar share a lot of similarities in that they're stringed and their setup, etc. but the science and value behind them is completely different. Bass defines the guitar's movements and is what connects the guitars to the drums. In a metaphor the guitars are the head and the bass is the neck and shoulders that it rests on. If you want to learn bass then get a bass and start at it because while playing guitar will help you familiarize the whole holding frets and strumming aspect, there are things that guitar won't be able to prepare you for. Besides, I don't recall there being any rule stated on when someone's prepared for one instrument over another, I've always decided when I'm ready for what I do and I've never had a problem with that.