Not sure what function you use to move the value, but if you add something like this to the custom js it will be copied when the form is saved:
function dffs_PreSaveAction(){
var txt = getFieldValue("NameOfTheMultiline");
var trimmed = txt.split(";\n").join(",");
setFieldValue("NameOfTheSingleLine",trimmed);
}
This will join the string with comma – change the join(“,”) part to use another separator.
Alexander