Nope, disagree. 10% weekly for regular maintenance, no more than 30% IF there’s an issue.Oh, definitely. Keeping two male bettas in the same tank is incredibly risky and takes a lot of knowledge and experience.
You should do a 40%-50% water change every week, including vacuuming the gravel. So, for a 10 gallon tank, you would take 4-5 gallons out every week, and replace it with fresh, dechlorinated water.