While the question is within the scope of SuperUser, it would receive a better response on the Unix/Linux Website
I wouldn't say so. While Super User's topics are probably more diverse due to Windows and OS X questions, we have tens of thousands questions for Linux, Ubuntu, Bash et cetera. People wouldn't ask here if this site wasn't a good fit for these questions.
Especially so that all Unix/Linux related questions can be assembled on one single website, making it easier for people to search for answers too
They could be, but you forget that these are all communities. If one spends a little time on these sites, they'll see that the communities are different from each other in so many other ways. It also depends on what kinds of questions are considered on-topic or not. For example, U&L might welcome historical questions that aren't based on an actual problem more than we do. On the other hand, if you know the users here,
I'll quote DMA57361♦ from the question you linked to:
The sites have different community spirits and different community goals, so it's important that if a user attempts to interact with one of these communities that we don't push them away. So if someone has made the effort to ask a valid question on SU, we will not automatically shunt it to another site.
So, regarding that, we're very well aware of the scope overlap—and this is something that sometimes confuses me as well from time to time—but then again, you're free to ask wherever you feel like.
Currently, if I have an issue, I could find the answer on either Unix/Linux or SU.
Yes. One might argue though that when you're using web search you'll land on either site anyway, so it doesn't really matter. Stack Exchange search could definitely be improved a little to either work across sites or pull in more relevant results from other sites as well, but if you're really looking to solve an issue you would probably extend your search to the whole web anyway.
All in all, I don't think we need to redefine scope other than what the FAQs of both sites say. Ask wherever you feel like, as long as your question is on-topic.