merge columns in 2 text files properly
I think I understand your problem: You are using Windows files on Linux.
Windows text files terminate by carriage-return+line-feed, while Linux text files terminate with only line-feed.
Therefore paste uses only the line-feed as end-of-line, thus conserving
the carriage-return. The paste result then seems to be in two lines
whenever that you display it.
To convert the end-of-line of Windows text files to that of Linux,
you may install and use
dos2unix.
Once converted, the paste result will be correct for Linux.