Two independent rhythms on an electrocardiographic recording

Two independent rhythms on an electrocardiographic recording

What are the situations in which you can have two independent rhythms on an ECG recording?
Two independent rhythms on an ECG recording can be seen in the following situations:

1. Piggy back (heterotopic) cardiac transplantation
2. Maternal and fetal electrocardiograms
3. Re-recording on a Holter cassette with incomplete erasure

The last one is not applicable now because Holter cassette recorders have become obsolete.