If the speakers are facing the same way, then it would make sense to have the positive terminals of the speakers connected to the same point on the amplifier module.
If the speakers are facing each other, this would seem to imply that the sound waves would cancel each other out, to a certain extent, so it might be better to change the polarity of one of the speakers so that they are anti-phase and hence increasing the strength of the output.
At the risk of starting a discussion about oxygen-free, copper-less, gold-plated negative-ion speaker cables, does this sound plausible
I realise the inside of a metal tank is not the best place for hi-fi audio, but "Every Little Helps", as they say.
Adrian.