As Charlie indicates, your Auxiliary Negative* Busí are OK. In fact, itís normal good practice to provide a Neutral Bus Bar at the Distribution panel (such as your Secondary Bus appears), and cable it back to a Main Ground Bus.
The objection is to utilizing a single
(shared or common) Negative Conductor to serve more than one Positive branch circuit conductor.
* Like the Neutral Conductor on shore-side AC systems, DC Negative Conductors are not Ground wires, even though they are grounded. Hence they most properly called Negative (return) conductors.