The main difficulty was to find a reliable datasource for the exercice. At the first time, after getting the pairs using the REST API, I sorted the pairs by last update to use the two most recently updated ones from this call.
It worked for some time, but at some point, I restarted the program, and I received no more message from the websocket, and so did the test APi in your documentation using the provided forms.
Then I decided to used two fixed pairs, maybe involving some well known currencies, like BTC -> ETH and BTC -> BAT. It worked at first, but then again, stopped working while returning a 500 error, even in the browser.
So I ended up choosing two random pairs, and use trial and error to reliable streams of data.
Also, one observation the return object of this call is not documented, you have to infer it from a test response, while the Order Book L2 socket message is well documented. However, websocket error messages, like 500, deserve a more complete documentation.
Finally, one little detail, the subject is not really clear about whether the datas should be cleared every 15 seconds or not, I made the educated guess that it had to, though I might be wrong.
I had to rush it a little bit in the end due to the timeframe, but it was fine, but I wish I had more time to do some unit tests.