The phrase "and difficult to separate" is correct and usable in written English.
It can be used when describing something that is challenging to distinguish or divide from something else.
Example: "The two concepts are closely related and difficult to separate in practice."
Alternatives: "and hard to distinguish" or "and challenging to divide".
Popular and unpopular provisions of the law are intertwined and difficult to separate.
This is why the lanthanides are chemically similar and difficult to separate and why they occur together in various minerals.
And now that the reality around designers is more controlled and difficult to separate from their comic-book images (almost by design, one suspects), finding the human dimensions is that much tougher.
However, H3PW12O40·19H2O is soluble in methanol and difficult to separate from water or methanol.
"The problems, challenges and dilemmas that organisations face today are often multi-layered, interconnected, chaotically organised and difficult to separate.
Members of the Akodon boliviensis group, including A. caenosus, are generally similar and difficult to separate, but they differ in relative cranial measurements and some other characters.
Sexual conflicts are notoriously difficult to study as the evolutionary forces of males and females often balance each other under natural conditions, and are difficult to separate from other evolutionary forces on male female interactions (Rowe and Day 2006).
Experience and cumulative volume are highly correlated and thus difficult to separate.
These tiny animals are difficult to see, often smaller than the particulate matter that comprises the substrata they inhabit, and thus difficult to separate from sediment, detritus and non-target species.
The one exception was for ductal and lobular carcinomas, which were intimately mixed and therefore difficult to separate in this first-look analysis.
Thus, discussions of epigenesis and preformation often bring in other ancillary questions and are difficult to separate from their contexts.
