How can you concatenate two huge files with very little spare disk space? [closed]

Answer 1

Two thoughts:

If you have enough physical RAM, you could actually read the second file entirely into memory, delete it, then write it in append mode to the first file. Of course if you lose power after deleting but before completing the write, you've lost part of the second file for good.

Temporarily reduce disk space used by OS functionality (e.g. virtual memory, "recycle bin" or similar). Probably only of use on Windows.