Impossible. How could Crono and Marle and even Lucca for that matter, have survived? Miguel basically implied that they were dead.
If Crono and Marle did survive somehow, then how do you explain the "spiritual" presence of Crono, Marle, and Lucca at Leene's Bell (Time Crash ground zero) and at Opassa Beach?
I would like to believe that they are still alive somewhere, but all evidence leads to their death somewhere between 1000AD-1015AD...