The Zohar, Chapter “VaYigash (And Judah Approached),” Item 126: “There is a firmament above the firmament, and this firmament controls them, meaning three firmaments where the right and left are one above the other,” meaning three firmaments, where the left and the right are above one another and the middle one “governs them, subdues them, and mingles them in one another”…"Under the firmament their wings were straight" since they all governed what they were appointed over, since the middle line establishes the illumination of both of them together, where the right shines from above downwards and the left from below upwards. This is why the right wing is equal to the left wing….
How is possible to measure whether the right and the left lines are equal? Can they be measured by quality, weight, and power? They have completely different qualities, and we don’t have any means to evaluate them according to an external system of measurement, like that of our world. “Equal” means they complement each other in such way that each of them provides all that is needed for the other. Hassadim from the right line fully supplements Hochma in the left line. By the same token, Hochma shines in Hassadim as much as it is able.
This means that they are equal. Thereby, they are partners, and each bestows all it has so that a mutual connection, called the middle line, exists in an optimal form. This is why they are considered equal. Perhaps, Hassadim supplies 90% and Hochma only 10%, and on the next level the ratio may be 70% and 30%, or 99% and 1%. It is called equal because one fills the other to the best of its ability. They are equal because they hold the same intention, thought, and calculation. The ratio between them doesn’t necessarily need to be 50/50. In spirituality, equality is when I fill you up with all that you desire from me, then we are equal.
From the Evening Zohar Lesson 6/9/10