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 behavior 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 the observable becomes dependent on collision conditions and, strictly speaking, cannot be considered anymore as a strongly intensive variable. For comparison, the results of the calculation of the considered observable with the PYTHIA event generator are also presented.