whitout using awk / sad and so on?

join will operate on lines in a set of files and create a resultset, made of lines from separate files. Is that what you mean?

cat file1.txt | grep "selection" > new_file.txt
cat file2.txt | grep "other_selection" >> new_file.txt

Of course, you can use awk instead of grep and awk or sed to change the content of the files.
Or, if you don't want to make any change/selection, cat file1.txt file2.txt >> new_file.txt

vimdiff.

Two files will be displayed side by side with highlighting of the differences. And you can 'do' and 'dp' to copy the diffs to and from the buffers, so this is selective. The best way to do this I guess.