Instead of providing separate windings for each output in a multiple output supply, a common technique for winding multiple output secondaries with the same polarity sharing a common return is to stack the secondaries one on top of the other. This is illustrated in the figure below. The windings for the lowest output provide return and part of winding turns for next higher output. This reduces total number of turns and also improves load regulation.
Illustration of an AC stacked secondary circuit.