Given that I get around primarily by bike, I figured a 2nd bike should back up my first bike in terms of its primary function: getting me to work and around town. Fortunately that's not a tall order given that most of my destinations are less than 10 miles away. I have a folding bike that fits the bill, and it's my current back-up bike. Lately I've been thinking of another bike, and I decided that the way to approach it was to figure what I couldn't do with my current bike, and make that a priority. For me, my current bike is not very speedy, nor does it do particularly well off road. I thought about a zippier, road bike, but decided it probably wouldn't get much use. I often have the opportunity to lighten my current bike, and yet I never do. On the other hand, there are a few times I take my current ride off road even though it is ill suited. And there are times I avoid more direct, off-road routes because my bike doesn't handle those situations as well. So my thoughts on a new bike lean towards something that doesn't mind going past where the road stops. Probably a fat tire bike because they just look fun.

So that's my thought process: What about my current bike needs to be duplicated to keep me getting to work, and what could a new bike do that my current bike could not? I agree that riding for a while could help you make your decision. The first bike that I bought new turned out to be almost entirely unsuitable to how I like to ride, but riding it informed me on what I should be riding. My next bike has been about as close as possible to being my perfect, one bike. But it's also shown me where a 2nd bike could really expand my riding options.