paste -d '\n' a.txt b.txt interleaves two files. It works, and it depends entirely on both files being in a corresponding order that nothing enforces.
Unequal lengths fail quietly
paste pads the shorter file with empty lines rather than erroring. The output has the expected structure and the wrong content. Compare wc -l on both inputs before running it — that check costs nothing and catches the failure that matters.
Any upstream sort breaks the correspondence
Positional pairing survives only as long as nobody reorders either input. A sort, a filter, or a deduplication applied to one file and not the other silently destroys the alignment, and nothing downstream can detect it.
Prefer joining on a key
If both datasets share an identifier, join on that field instead. It is order-independent, it reports unmatched rows rather than hiding them, and it cannot produce a silently misaligned result. Positional interleaving is acceptable for a quick one-off where you can eyeball the output; it is a poor foundation for anything repeated.
Try it: Interleave Two Lists on SeoWolf's Notepad.