Multiplicity fluctuations in two separated pseudorapidity intervals are studied in terms of strongly intensive variables. This work presents results obtained in the MC model with quark-gluon strings as objects extended in pseudorapidity space and acting as particle emitting sources. The string interactions are implemented within the discrete model of local string fusion. Strings of fluctuating length in pseudorapidity are considered in order to estimate the non-critical background of fluctuations. The results on the dependence of strongly intensive observables on the width of acceptance windows, on the pseudorapidity separation between them and on the string density qualitatively describe the available data from the NA61/SHINE and ALICE experiments.