How to run 'join' command for multiple files (of 2 types) in a folder
If all of your .tf.txt and .tt.txt files are in the same directory, the following command should work.

for tffile in $(ls *.tf.txt)
do
    prefix=$(basename $tffile .tf.txt)
    join ${prefix}.tf.txt ${prefix}.tt.txt &gt; ${prefix}_final.txt
done