Two posts, two different instances, two different sets of comments. Ok, there are pros and cons:
Segmenting the conversation means that different opinions and perspectives have a bigger opportunity to rise to the top in each discussion. That’s good.
There is also a risk of too few different viewpoints being present in one instance or another, making the possibility of an echo chamber forming higher. That’s bad.
I’m not suggesting that there’s a single perfect way to handle it, nothing is that black and white, but maybe there is a chance to do better. Perhaps crossposted content could contain a second link underneath between the body of the post and the comments section that could inform users that an identical post existed on another instance. Perhaps the comment sections could be merged but the user could choose to filter by instance if they wanted to.
lemmy-ui actually groups them together. It all depends on the app / frontend implementation that you're using.
Here is how the OPs example shows up with all the x-posted communities listed.
I have noticed, though, that two posts of the same URL within a single community are also listed as x-posted, and the "duplicates" don't show up in the community's feed at all. Which can be problematic if you're linking to a page with a static URL that changes over time (such as sports league statistics). The trick so far has been to highlight text that has changed on the page and get a link to the highlighted text.