Do we know which of the two sites was selected yet?
Source: I work in one of the physics labs that is pushing ET.
LIGO, which made the first gravitational wave detections, is located in the US and also has a pair of detectors (one in Hanford Washington; the other in Livingston, Louisiana).
The reason to build in pairs is because the detectors need to be _extremely_ sensitive in order to detect gravitation waves. Such sensitive detectors can easily pick up all kinds of noise from local phenomena (such as a truck driving nearby), so having two sites separated by a good distance helps rule out local noise. Only signals that are replicated across both sites need to be evaluated.
There are other benefits to have multiple detectors, too. Having two detectors gives us some ability to determine the direction the wave is coming from. More detectors will allow us to determine the direction even more precisely.
It’s a cost benefit analysis (and one I’m not remotely equipped to make). My guess is they went with two sites because that was the smallest number they could get away with while still being able to make detections.
[1]: https://en.wikipedia.org/wiki/Einstein_Telescope#Current_des...