AdditionalOutput.java

final TupleTag<String> dataTag = new TupleTag<String>() {};
final TupleTag<String> errorTag = new TupleTag<String>() {};

PCollectionTuple mixedCollection =
  names.apply(ParDo
    .of(new DoFn<String, String>() {
      @ProcessElement
      public void processElement(ProcessContext c) {
        if (c.element().startsWith("Foo")) {
          c.output(c.element());
        } else if (c.element().startsWith("Bar")) {
          c.output(errorTag, c.element());
        }
      }
    }).withOutputTags(dataTag, TupleTagList.of(errorTag)));

mixedCollection.get(dataTag).apply(...);
mixedCollection.get(errorTag).apply(...);