Connecting of the generators in series before the rectifier is a tricky methode. Assume that the three phases of generator 1 are called U1, V1 and W1 and that the three phases of generator 2 are called U2, V2 and W2. Assume that every phase generates a sinusoidal fluctuation of the voltage. So the voltage has a maximum at a phase angle alfa = 90° and miniumum at alfa = 270°. Assume that both generators are identical and that they are mounted on the same shaft. This must be done such that a positive voltage peak in phase U1 corresponds to a positive voltage peak in phase U2. This is realised if the north poles of generator 1 are in line with the north poles of generator 2 and if the coils U1 are in line with the coils U2. Only if this is the case, both phases U of the two generators can be connected in series resulting in doubling of the voltage. If the positive voltage peak of phase U1 would correspond to a negative voltage peak of phase U2, the final voltage will be zero. Any of the three phases can be called phase U if both generators are identical.
If each generator is rectified using a 3-phase rectifier, one can connect the DC currents in series but it must be done such that both currents have the same direction. For connecting of the generators after the rectifier it isn't necessary that the north poles and the coils of both generators are in line. However, a rectified 3-phase current has a small fluctuation of the voltage (see figure 9 in my public report KD 340) and the magnets and coils can be positioned such that the fluctuation of one generator is partially neutralised by the fluctuation of the other generator (if there is a phase shift of 30°).
