This is yet another example of streaming, using the 'galloping', or 'ABA' rhythm paradigm. The two possible perceptual states of this sequence are illustrated in the figure below (this is Fig. 6-11 of the book). When the repetition rate is slow, or the interval between the two tones is small, the sequence is usually perceived as a 3-note melody with a galloping rhythm. At faster rates, or when the interval between the two tones is large, the sequence breaks down into two streams, the upper one at half the repetition rate of the lower one.
You can experiment with these sounds below. Start at slow repetition rates and small frequency intervals, then move to faster rates and larger intervals. Try to locate the combinations that result in the perception of two streams. Can you hear the streams separate and merge back?