commandliners

$ diff -W80 -y one two
this is it                            | this si it
and this is that                        and this is that

[The relevant option is '-y']. Shows all the lines of both files in parallel (line by line) and places a vertical line ‘|’ between them when the corresponding lines differ. The -W option sets the ‘line width’, each file gets half its value (so if you want to see long lines completely, you need something like -W161 or so).