On my basses with this option, I usually come down to liking the sound of either both in parallel (or both in single-coil, if that is an option), or neck pickup in parallel/single and bridge in series. This is purely for my own taste, though, and YMMV. But my observations are that both in parallel/single gives the most air and openness on top. But, running the bridge pickup coils in series gives a bit more heft and low end output to the bridge pickup, which sometimes makes for a positive tonal shift (and sometimes is not needed).
To further clarify and limit these remarks, I am predominantly a fingerstyle player and I play mostly over the neck pickup. If I put the neck pickup in series mode, it slams like a mofo, but sometimes, it darkens/muffles the top end a bit more than I'd like.
IME/IMHO, and YMMV!
Tom.