You can add this snippet to your Custom JS to have a check when saving the form:
function dffs_PreSaveAction(){
var pp1 = getFieldValue("PeoplePicker1");
var pp2 = getFieldValue("PeoplePicker2");
if(String(pp1) === String(pp2)){
spjs.dffs.alert({
"title":"ERROR",
"msg":"You cannot use the same person as Owner and Approver"
});
return false;
}
}
Alexander