From man diff, you can use -y to do side-by-side.
-y, --side-by-side
output in two columns
Hence, say:
diff -y /tmp/test1 /tmp/test2
Test
$ cat a $ cat b
hello hello
my name my name
is me is you
Let’s compare them:
$ diff -y a b
hello hello
my name my name
is me | is you