If I use multiple heatsinks in the same enclosure, does the thermal resistance at airflow of each heatsink change?
Each heatsink will have the same thermal resistance to the air in the enclosure.
What will change is the temperature of the air. Say for instance the airflow crosses one heatsink and then the second. The air will be heated by the first heatsink and then run over the second. So for your thermal calculation you will need to take into account the fact that the air is warmer as it hits the second heatsink.