  The double-energy double-velocity ($2E$-$2v$) method allows assessing
fission-fragment mass yields prior to and after prompt neutron emission with
high resolution. It is, therefore, considered as a complementary technique to
assess average prompt neutron multiplicity as a function of fragment
properties. We have studied the intrinsic features of the $2E$-$2v$ method by
means of event-wise generated fission-fragment data and found severe
short-comings in the method itself as well as in some common practice of
application. We find that the $2E$-$2v$ method leads to large deviations in the
correlation between the prompt neutron multiplicity and pre-neutron mass, which
deforms and exaggerates the so called `sawtooth' shape of $\bar{\nu}(A)$. We
have identified the treatment of prompt neutron emission from the fragments as
the origin of the problem. The intrinsic nature of this deficiency, risk to
render $2E$-$2v$ experiments much less interesting. We suggest a method to
correct the $2E$-$2v$ data, and recommend applying this method to previous data
acquired in $2E$-$2v$ experiments, as well.
