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Most of DC power supplies sold (but not all) are floating. Meaning that there is not DC connection to the earth. Some especially the ones with earthed metal chassis have high value resistor and small capacitor in parallel connection to the chassis to reduce high frequency noise. For most of purposes they can be regarded as floating.
To find out if your PS is floating or not - use multimeter! Switch it to low Ohm range – buzzer or diode test. With PS not connected to power connect one probe to common, negative or – (whoever it is called on your PS) terminal of your PS and the other probe to the Earth pin of the mains socket on your PS or (in case of fixed power cord) to the Earth pin of the mains lead. If your multimeter reads open circuit – your PS is floating.
Now if you have a number of single output (meaning – 0 +12V, 0 +13.8V, 0 +6V and so on) floating Power Supplies it is safe and preferable to connect all 0V together. It will not matter if those PS are plugged to different Power points. But it must be done with PS being physically as close to each other as possible (side by side). Running a meters of wire to connect one PS to other would defeat whole point of exercise.
Unless you know what are you doing do not connect multiple outputs or Negative zero Positive PS together.
Personally I would not use non floating Power Supplies for my astronomy gear and regardless what I said about PS the Battery is the best power source especially for cameras as it provide cleanest DC.
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