[DonRotolo]

If the output shafts are supported at the far end, a lot of the bending moment around the angle bracket would disappear. And of course a bar or plate across the top, from one edge to the other, including the tops of the gearboxes, would add considerable rigidity.

There's an engineering problem there, what kind of plate or bar would be optimal (stiffness vs. weight)? Note that it does not need to be of constant cross-section, but beware of creating an "slender column" in compression. I'll leave that as an exercise for you.