One Twin with Down Syndrome risk?

If you are pregnant with Twins can genetic testing pinpoint which Twin is likely to have Down's Syndrome?
Also: If so (one Twin has Down's but the other doesn't) can Doctors perform an abortion on one and leave the viable baby to go to term?? How dpes this work?
Btw I'm not in this situation! It's just a hypothetical question. I'm curious sorry if its an offensive question.