I am by no means an expert but I think Family Tax A everyone can claim and Family Tax B is generally a supplement when only one of you works or you are low income. Both are income tested and I don't think you claim one or the other. I think everyone gets FTBA and then FTBB is a supplement on top of that if that makes sense.

There is an estimator online that will tell you what you are entitled to ... https://secure.centrelink.gov.au/Rat...OriginName=FAO