Theoretically, you are fine with 2 hours connecting flight. My partner and I take SQ12 often and then connect to Air Canada which is in a different terminal. When SQ12 lands usually the immigration line is quite long. The longest waiting time from my experience is about 60 minutes (I am in a visitor line). Sometimes the pax with visa waiver program get a different line, so much faster. The customs can be quick if you don't get stop which generally they stop me because of my passport. If you check through your luggages from SQ12, there is a drop-off area for the luggages for your connecting flight. Generally they will ask you to unlock your luggages part of the security screening. Then heading upstair to LH check-in counter to get your BP.