Thank you for your consult at DoctorSpring.com.
For administering a paternity test, 1 ml of amniotic fluid is required.
For the genetic testing too, a small amount is required.
Sample for the DNA paternity testing can definitely be withdrawn at the same sitting for amniocentesis. IT does not need two sittings - the sample for the paternity testing can be directly transported to the concerned laboratory.
You should insist that this be done, if you want a paternity test.
There is no added risk over and above the inherent risk involved in the procedure of amniocentesis.
Please feel free to ask for further clarifications.