Hi apeeet,
It is possible to compare to reports in HFSS. Assuming you have to compare project1 and project2. Here are steps
1) open project1 rectangular plot, right click to get option called export (see the screen shot), select any one of the format such as *.csv, *.tab, or *.dat and save it. (better save it in *.csv format). save the file.
2) Now open your project2 report (rectangular plot). right click on plot to get option named import, select your project1 data
3) Both results can be compared now