They need to be under responsible charge of a PE. Generally the PE firm should hire the 1099 contractor (not the other way around), and the PE should have full ability to comment, revise and direct the 1099 contractor as if they were working as a junior staff under the PE. As long as that is the chain of command, there shouldn't be any issues. I have seen some comment here that their state requires W2 employment for responsible charge, but the 18 states I work in do not. Some state word it weird, but generally the engineering board does not have jurisdiction to require W2 employment for responsible charge.