Hi newbie, is there a particular reason why you would want to do an amnio?
I am also pregnant with twins (16wks) and did consider amnio for various reasons - but the risks do seem a lot higher (up to 5X more risk of miscarriage than amnio during a singleton pregnancy) so I decided against it. I made the decision after the 12wk ultrasound & bloodtest, which showed a low chance of Down Syndrome.
That said, if you really want to be sure, there are plenty of dr's here that regularly perform amnio's on twins so should be possible to find someone experienced if you wish (eg Dr Ghosh, Dr Doo)
Good luck! When are you due?