Someone once told me I should know guitar first. Are these instruments different enough that learning banjo is independent of knowing guitar?
I play both fingerstyle guitar and clawhammer banjo, and aside from basic left-hand fretting technique, they are very different. Here are some ways in which learning the guitar first will actually make learning the banjo harder, not easier:
This isn't to say you can't do it. I'd been playing guitar for almost twenty years before I picked up a banjo, and with practice, I overcame all of these obstacles---and I love them both. But if you're only interested in the guitar as a gateway instrument to the banjo, I'd say don't bother. Go straight for the banjo.
There are common elements between both elements, moreso than between a cello and a flute for example, but there's absolutely no reason you should know guitar first. Experience with guitar would probably help to some degree with banjo, and experience with banjo would probably help to some degree with guitar. But that assumes prior experience.
If all you want to learn is banjo, then learning guitar will be a waste of your time. You can learn banjo most effectively by learning banjo. Spending a year on the guitar might make you better at banjo than someone who has never played an instrument, but it definitely won't make you better than you could be if you played banjo for that year instead.
Now, if you have decided to practice banjo exactly 1 hour per day and are looking for something else to do in your free time, adding guitar wouldn't hurt (assuming you don't strain yourself). But decreasing banjo time in order to play guitar will result in worse banjo playing in the end.
Disclaimer: I play guitar but not banjo.