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Hi do you mean parallel or series? Series is what allow the voltage to be multiplicated. WHat do you use to connect to the transformer and to connect to the wood (bolts ,clamp?),
Parralel is side by side, series is one at the end of the other. if you use series, it is possible that you exceed the voltage tolerance of the second transformer and it can form little arcs breaking trough the dielectric insulation (the enamel around the wire inside the transformer).
ALso at those clamp, carbonization may occur with that kind of voltage. Contact cleaner or rubbing a file on them may help in giving them good contact again.
If you use too thin cable, copper may melt and stretch, that would increase the resistance of the wire, make sure you use tick ones.
You may also have changed the brand of the bicarbonate sulfur you use (i guess this is what you use for your conducting solution). Some may be more effective than other, you may also need to use fresh one if the pack you have is very old.