  The strongly intensive observable between multiplicities in two acceptance
windows separated in rapidity and azimuth is calculated in the model with
quark-gluon strings acting as sources. The dependence of this observable on the
two-particle correlation function of a string, the width of observation windows
and the rapidity gap between them is analyzed. In the case with independent
identical strings the model calculation confirms the strongly intensive
character of this observable: it is independent of both the mean number of
string and its fluctuation. For this case the peculiarities of its behaviour
for particles with different electric charges are also analyzed. In the case
when the string fusion processes are taken into account and a formation of
strings of a few different types takes place in a collision, this observable is
proved to be equal to a weighted average of its values for different string
types. Unfortunately, in this case through the weight factors this observable
becomes dependent on collision conditions and, strictly speaking, can not be
considered any more as strongly intensive variable. For a comparison the
results of the calculation of considered observable with the PYTHIA event
generator are also presented.
