It just occurred to me that if there is only one water connection between the two and it is only 1/2" and has a t in it. There would be a slight vacuum from the line going to the pump after it has been primed. That slight vacuum is likely enough to cause a Venturi effect in the T to prevent the movement of fluid between the two tanks. And if that T is closer to one tank then the other it would draw more from one then the other.
I could be way off base, because I'm not sure it it was said weather or not there are more than one water line between the two tanks. And I am sure there are some highly skilled engineers that tested the design, but it sound possible to me.