Nothing states that they are dead(k, Ayla and Frog died prolly because of age), but maybe, Crono and Marle are fighting Porre on the Zenan main land, even after guardia fell. And for an alternate reality, you need a dimension split, which just occured once in CC, so there are just 2 "realities", but i don't know if you can call it alternate reality, it's just a diffrent dimension(2nd time spelled wrong).