See these two questions. They are very much similar to each other. I was shocked to see both these questions hit the HNQ.
How does Stack Exchange allow two similar questions to hit HNQ?
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It's an algorithm that chooses these questions, every 15 minutes. If a question has the necessary "hotness" (for simplicity, assume that it has multiple answers and both the question and answers have several upvotes) and isn't prevented for other reasons, such as the questions per site limit or having a blacklisted word in the title, it's added to the list. We don't do any checks for content between various questions, particularly not between two completely different sites.
So... it's less that we "allow" it and more that we don't prevent it... and, anecdotally, I don't think it happens that often.
In your specific case, this is true. The Personal Finance question has a score of over 50 and three visible answers, with the top answer scoring over 60. The Economics question has a score over 30 and three visible answers, with the top answer scoring over 40.
Both questions - as you might expect - are under 30 days old and, based on their ranking on that page, are the "hottest" questions on their respective sites.
I'd also add that blocking one would damage the primary purpose of the HNQ List - to encourage movement around the Network and visibility of Network sites. If both sites have great questions and answers about this subject, there's no reason both shouldn't get the visibility.
You can read more about it here:
If those questions were on the same site, one would have been closed as a duplicate of the other and therefore not be eligible for the HNQ. There’s also a limit to the number of hot questions a single site can have at once. But neither of these are applicable here.
The HNQ also blocks some questions based off their titles, but again that would not apply here. Only titles with rude/vulgar words or MathJax are blocked, IIRC.
Also, being on two separate sites means that the initial (ie pre HNQ) audience for each was probably mostly different people. This means that most people were voting like each question was new and unique, because they hadn’t seen the other question on the other site. Getting a lot of votes quickly is the basic way that a question gets into the HNQ so it makes sense.