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Its most likely due to one of the power supplies is a Double insulated type.
You will have a differential voltage from the two supplies due to a different earth. The double insultaed has no earth, and thus when you reference it from a earthed point, you will see a voltage. I have seen voltaes as high as 200V.
You will need a proper earth before you connect the supplies together. This will then connect the 0v or earths together, and supply correct levels.
One easy way to tell if you have double insulated supply, is to check the AC cable for the earth pronge, if its not there its a Double insulated type. Another is to do a Ohms mesaurement between both earths. It should read very low ohms, like several ohms at most.
Theo
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