See this answer for several pictures of Farside.
When the third stage is finished (or nearly so), the fourth stage is then ignited, and it literally shoots out of the third stage in which it is embedded.
Thus my use of the word "parallel stages". They are oriented in parallel but fired in series as separate stages.
Question: How common are "parallel stages" in a launch vehicle? What are some other examples for launch vehicles that reach space, or is this the only one?
note: It doesn't have to be orbital, another sounding rocket example would be fine.
Images from this answer, click for full size or view there.