For Growth Through Sacrifice, you check the threshold as soon as you face the choice between "add a Presence" and "remove a Blight" (to see if you can do both). If you don't have 2 Sun at that point, it doesn't matter if you get 2 Sun by adding a Presence, the time for checking "can you do both?" has passed. (Note that the ordering of the two options is irrelevant to this.)