For illustration, here's a 9:1 unun construction using 3 wires from M0UKD:
Since the A and B windings are connected together in series, there's no compelling reason you couldn't replace them with one longer wire that has the same number of turns.
For an ideal transformer only the number of turns would matter, and how those turns are arranged would be irrelevant. In practice how the turns are wound affects things like the parasitic capacitance: this is why you can "tune" an inductor in a filter a little by pinching or spreading the turns. Whether this change makes the device better or worse for your application will depend on details like the frequency, core dimensions, wire gauge, and so on.
I've read that crossing over the middle of the core as in your question is supposed to reduce the capacitance between the input and the output. My guess is it makes negligible difference in practice. I like that it makes the wires come out on opposite sides of the box and it looks neat. Easiest way to find out is to build it and measure empirically.