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Just to add my ten p's worth, I once connected two 10amp power supplies together and expected to get 10amps, I switched them on and immediately blew one up and it was not even loaded! These were sophisticated supplies and this can happen if there are two monitoring circuits trying to correct each other so be warned (it did make quite a large bang). I assume that most PSU's are of the "Switching" variety and would both be trying to regulate each other.