First some comment concerning your code snipped:
You are creating a new record with the ID value "new" + dt.format('U')
and in the next line you are setting this ID to 1? You should create the record with the appropriate (and unique) ID. Otherwise you may override a existing record.
Now your problem:
You are adding a record with the same ID twice. So for the store there is only one record. Either don't assign a ID value at all and let ExtJS take care of it or ensure that each instance get it's own unique ID. And create a new instance each time!
You need to do something like this:
var rec1 = new store.recordType({}, 1); // remove the 1 with your own unique ID value
var rec2 = new store.recordType({}, 2); // remove the 2 with your own unique ID value
store.insert(store.getCount()-1, [rec1,rec2]);