As per both request, and author confirmation that this does clarify parts of the question, I am reposting my comment as an answer;
ultimately, I believe the two tags are quite different from each other, and there is a distinction between the users who answer them. They should not be merged together.
The two terms are quite different from each other
Initially, I had never heard of a texture atlas, before. I have had some experience with spritesheets, but I had never even heard of the term texture atlas. I did not have to look far for a good definition; Somebody has already asked "what is the difference between a sprite sheet and a texture atlas?", here on the main site. The answer quite clearly defines a distinct difference between the two terms, and the author even comments on the asker's presumption that they are both the same.
"A sprites-sheet (often refers to a large image that) is supposed to contain the animation frames of a specific 2d character or projectile in a game." ... "A texture-atlas (is often taken to mean a large 2d image that) contains many textures."[sic] - Answer submitted by Zehelvion
"Everything is that same, bits containing ones and zeroes." - Further comment submitted by Zehelvion
I can see how there are some disctinct similarities between the two terms; but I would not use the term "spritesheet" to define a texture atlas, and I would not use the term "texture-atlas" to define a spritesheet.
There is a clear distinction between our top users
If we look at our top users for both spritesheet and texture-atlas, we see a distinct set of different users (with the exception of Kromster, who is in the top three for both. Excellent job!); this tells me that you very well can be an expert in one, and not the other - at least, within the scope of this website.
What further improvement could be made?
Can we make a further improvement?
Yes. It is clear that the two tags have similarities, though not enough to warrant merging them within the scope of this site. Perhaps we could improve usability by writing up a guideline for the texture-atlas tag; at present, it has none. A guideline has now been published to the texture-atlas tag.