I would breed them both this year, they kid in spring 2017, the AN put in kid again autumn 2017, the BSx should be able to run through better, not sure if AN's run through, I don't keep them,, but from then on you could try alternate years. Yes, their female progeny would be ready autumn 2018,
To me pure bred means Toggenburgs or Sanaans that go back to he imported stock, no impurities allowed, if you mean your BS, herd book registration, i think its about 4 generations, I'm sure someone else will be along soon with that, have you a registration for your BSx?
Are you in the BGS?