You need to append the new element to existing element's parent before element's next sibling. Like:
var parentGuest = document.getElementById("one");
var childGuest = document.createElement("li");
childGuest.id = "two";
parentGuest.parentNode.insertBefore(childGuest, parentGuest.nextSibling);
Or if you want just append it, then:
var parentGuest = document.getElementById("one");
var childGuest = document.createElement("li");
childGuest.id = "two";
parentGuest.parentNode.appendChild(childGuest);