To what extent similar remarks from the same author can appear in different papers? I came across this situation that seems not immediately obvious as regards the "line" for self-plagiarism. 

Suppose I have two papers on two related problems such that merging the two papers is not an option. It seems important to draw some remarks, which may just be qualitative descriptions in a novel way but which may be helpful in clarifying concerned things in the nonempty intersection. By nature of the problems the intended remarks would be similar to a certain extent. In this case, do I have to deliberately make them look sufficiently different? Otherwise, do I have to cite the remarks I made? I would love to know the predecessors's wisdom about this matter. Thank you.
To some extent plagiarism isn't charged when there is essentially only one way to say something. But this rule mostly applies to things that are widely known, not research results. This may be the case here or not. 

But it is better to cite the other (first) work in subsequent work (or cross cite if appropriate). One of the purposes of avoiding plagiarism is to allow readers to have access to the complete context of an idea. This might be especially useful in the case at hand, since the "similarity" itself may provide context that would be helpful. 

When in doubt, cite it.