Series give you 18V - PARALLEL gives you 9V. Two batteries in series INCREASES the drain and will last shorter than one. And it's not a good idea to parallel batteries as if one goes it takes the other with it - and the weaker battery will be charged from the other and possibly leak. When I converted to 9V from 18V I disconnected the second battery holder and just use it to hold a spare.
FWIW EMG provides these powering tips and diagrams, and warns nothing about hazards either series/pararell 9v/18 v:
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I think your hazards comes from all other wirings with batteries, like car batteries etc, mixing different brands, and alkaline with lithiums and so on. Once you do a series-pararell connection which means BOTH doubling it to 18v (which means 4 batteries) and doubling capacity you might get some minimal risk of thermal runaway, but I have never ever experienced it with small 9v batteries, that are of the same brand, and hopefully from the same batch. If you mix with one 3 v battery and one 9 v to get 12 volts I can very well think that things will happen. As long as the aH (ampere hours) and the voltage are the same there's little harm. Very little. At most unlucky circumstances it may leak, but it will not ever explode.
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