Those two narrow bands are the Right Ascension boundaries of the error box of the signal. Their heights represent the Declination boundaries. This WIKI article is very good, it is obviously written by an astronomer, you can trust it.

Although ET may communicate by some exotic method we may not be familiar with, the microwave spectrum is still a good place for anyone to put a beacon because astronomers of all species will be using it to map the interstellar medium, so someone will be listening.

In this region, there is very little background static, so the Signal to Noise ratio will be very good. Radio astronomers particularly like the Waterhole (everyone comes to the waterhole to drink, right?), there, in a deep dip in the microwave background, and between the emission spikes of the H (21 cm) and OH-(18 cm) radical. Those species exist in molecular clouds and if ET is into astronomy, he'll be monitoring those freqs. Of course, there are a lot of possible channels between those two, and many more throughout the bottom of the Waterhole. It is a needle-in-a-haystack problem.

I would suggest listening at any harmonic of any common emission spike in that region. Those spikes are the emission lines in the microwave spectrum we use to identify molecules in deep space.