A linear X linear interaction effect of X1 and X2 on a quantitative outcome variable Y can assume various forms. But, one feature of all linear X linear interactions is that predicted values from the fitted equation for different values of X2 converge to a single cross-over point at some point on X1, if predicted values are projected onto the (Y, X1) plane. Of course, predicted values for X1 converge to a single cross-over point at some value of X2 if predicted values are projected onto the(Y, X2) plane.
