With such low voltage, the resistance of the two lights in series, might be great enough to drop the voltage so far that the lights won't burn. So you can't run a wire from a positive supply, to one terminal of bulb A, then connect the other terminal of that bulb to bulb B, and connect the final terminal of bulb B to the negative supply.
In general low voltage DC circuits are always run Parallel. Run a wire from the positive supply to one terminal of bulb A, and also to a terminal of bulb B. Then run a second wire from the negative supply to the remaining terminal on each bulb.