Okay to Neutra flush crankcase twice?

[Nitrox]

My father's got 2 vehicles with over 80k miles of street stop and go duty. I ordered him 2 gallons of Neutra, and we were discussing whether it was okay to do the 500 mile cleaning with Neutra, and then do it again the next oil change. Is that okay to do, or would it be excessive? Considering Neutra is very reasonably priced, I thought it would be a good idea. Both crankcases hold 5 quarts of oil, so we're only talking 20 oz of Neutra for the 2 cleanings of the 2 vehicles.

Thanks.