  We present a direct comparison of the number of dynamically correlated
molecules in the shear-mechanical and dielectric relaxations of the following
seven supercooled organic liquids: triphenylethylene,
tetramethyl-tetraphenyl-trisiloxane, polyphenyl ether, perhydrosqualene,
polybutadiene, decahydroisoquinoline, and tripropylene glycol. For each liquid
we observe that the numbers of dynamically correlated molecules in the shear
and in the dielectric relaxation are proportional. We show that this
proportionality can be explained by the constancy of the decoupling index of
the shear and dielectric relaxation times in conjunction with time-temperature
superposition. Moreover the value of this proportionality constant is related
to the difference in stretching of the shear and dielectric response functions.
The most significant deviations from unity of this constant are found in a
liquid with strong hydrogen bonds and in a polymer.
