Firstly, FTB is only available to single income households, so double income families are not eligible for part B of the payment.
Secondly, if the government wants to ensure that the tax rates are based on household income rather than individual income, this should be done consistently across the community, not as special treatment for households with a SAHP. In this case we would have a standard household tax rate that applies to all households regardless of the number of income earners or whether they have children. So, for example, if you have 3 workers in the household their individual tax rates would actually be higher. Not sure I think this is a good idea, but it is consistent with what you are saying.