Brookie,
I don't think there is a right or wrong answer here. Use both if you need to have the overrun protection and only one if the spring is stiff enough to give it to you alone. On many of the vintage Perfects, it seems that the springs get tired. This is possibly due to them being put away under tension for long periods. That or maybe they just lose spring. For these, it helps to engage both pawls. I had a 50's 4 1/4 though that had such a good spring that I not only didn't engage both pawls, I also didn't fully engage the tension adjustment on the one.
I suspect that tension varies from reel to reel for other reasons too. I used to have two identical Bougles. On the one, one pawl was all I needed. On the other, both fully engaged was barely enough to prevent overrun.
sinktip