I personally feel its better to separate them.
I work in an environment where things like action names or pages are a like between the 2 platforms and can be exactly the same so it makes it more challenging if you want to report then you need segments. Imagine in report builder then you need to have extra segments vs just picking each reportlet and pickup just report suite.
Also if one env suddenly pollutes data then both platforms get hit.
From Adobes perspective there is no extra cost in setting up 2 report suites vs 1 as it boils down to server calls.