Yes he is right, if both were on one shaft it would work. Having wind turbines in series is a bit like solar panels in series with one shaded. With wind the problem is so much worse as both turbines never see the same wind at the same time.
I am not sure how your stators are made, if they are axial flux machines and potted in polyester resin it may be possible to salvage the wire. If potted in epoxy then probably not. If they are iron cored machines then it is unlikely that you will be able to reclaim the wire.
Even if you do manage to reclaim it, it may not be much use as you need twice as many turns of half the csa wire, it is very unlikely you will have room for twice the turns of the same wire. Unless the original was delta there is little way round it going from 12 to 24, you would have more options going the other way.
To be honest you will save a lot of effort and frustration by going for new wire. You need half the csa, if using the AWG system then 3 numbers less. Wind with twice as many turns or perhaps 5% less than twice as the diode drop and line loss will be less at 24v, especially if the original machines showed any hint of stall.
Flux