Why do we need to use special versions of voltage regulators on negative rails when the input is isolated from the positive rail?
I originally misread your question, or the part about separate isolated power sources was missing. Does the following schematic accurately describe what you are trying to accomplish? (It is always helpful to provide a schematic) If so, the answer to the question of why there exist the 79xx series of regulators, is so one does not need to have two truly isolated (floating, ungrounded) supplies.



simulate this circuit &ndash; Schematic created using CircuitLab