Running CM9 + Supernexus 5 : Dual boot
1) Root your GS2
2) Install Siyah v5 kernel (download the ZIP and install via cwm)
Link
3) This will enable your phone into dual boot.
4) Then you can install many roms as secondary and test.
Link
5) Finally install the one you like, as the main rom.
However, :1ign19: Before you start, read, read and read what you plan to do.
SGS2 is not like motorola defy which cannot be bricked. SGS2 can be 'really' bricked means everything gone and turning your phone into a mere paperweight.