Yes, it is possible to hold both an EASA license and a FAA certificate. There’s only one written test for the FAA ATP and most folks just do a few weeks of online studying and pass the test. There is no requirement for flight training for the ATP checkride unless it is also a type rating ride. Then o would have to complete a flight training course. There is also a requirement to complete an ATP Certificate Training Program before you take the ATP written exam. It’s a 40 hour class that includes 10 hours of simulator time. It’s designed to fill in the knowledge gap between a 1470 hr CFI and a 1550 hour regional jet co-pilot. It’s really redundant (to be polite) for someone who has flown in jets but that’s the requirement.
Yes, didn’t think where it’s posted.