Why can't you use the ground of a second DC power supply as a return?
You mean ...







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There is no potential difference established between the +ve of V2 and the -ve of V1. voltages are not absolute values, they are differences between two points.

If you measure the voltage with respect to the unconnected end you would see that there is no voltage across R2 - hence no current will flow through it.

If there is a "hidden" connection between the negative sides of the DC supplies, then a potential difference exists across R2.







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