I think it is a bit of a gray area for PV source circuits inside the PV array. Once the circuit leaves the array all the conductors and the EGC need to be in the same conduit.
Inside the array it is sometimes not possible to match up a positive with a negative conductor everywhere. Unmatched conductors run from module to module in a string and often an unmatched conductor has to be home run the full length of a string before it can be paired up with the other conductor. So for instance, if I were making up a string out of modules in different rows I might have to take a single conductor and put it in conduit to get it to the other row to pickup all the modules in a string. At some point I have to bring the positive and negative for that string back together before leaving the array.
I would say running all the positive conductors down one side and all the negative conductors down the other side of the array would be an unusual design and not one I would do, it's just not my idea of a best practice taking into account DC combiner locations and trying to equalize the lengths of the PV source circuits. Maybe for a small array it would work.