You probably mean that incomes taxes should be subtracted from wealth taxes? (I don't know that "deduction" is right technical term). That sounds like a good idea, and should probably also include subtracting out capital gains taxes as well. So if you had a $500,000 calculated income tax liability, and a $600,000 calculated wealth tax liability, you would only end up paying $600,000 in taxes (instead of $1,100,000).

  $tax_paid = max($income_tax_liability + $capital_gain_tax_liability, $wealth_tax_liability)

...that does seem like it would seem to alleviate PG's concerns about adding "a mere 20%" to the income tax rate.