What do you mean by they add? they add but with no use?
I don't understand your question.
If I have an economic starbase with all of the standard production/research increasing modules (DA, or TA Terran tree), it grants +24% production/research to planets within its sphere of influence.
If I add a second one at 24%, combined they now grant +48% to planets within the area that is inside both of their spheres of influence. If they're adjacent, this can be assumed to be essentially the same sphere.
Does that help?