@hmacdermott -
Obviously you have a reason for wanting to know the decision process for which one is dropped. Fortunately, it does not affect the grade. (based off of what I recalled after writing this statement, it does affect the grade if the assignments are of different values)
I am not sure of the order in which a low score will be dropped. It could be the first one attempted, it could be the quiz that was created first, it could be the first one alphabetically, it could be based on the one with the most points available, or it could be the one with the least points possible.
In writing the above, I triggered something I already knew and had thought of. Everything above is not what is done. The dropping lowest issue has been discussed in the past, and below is a recap using your example.
Canvas will drop the score that results in the lowest score over all for the assignment group. If all the assignments are worth the same number of points, then it is one of the criteria I mentioned above and it will not matter. If the assignments are worth different points, then my statement 2 sentences ago is what decides which assignment to drop.
Example: 4 assignments in a group where the lowest score is dropped with the following scores on assignments
#1 - 10/15
#2 - 7/12
#3 - 0/10
#4 - 0/5
In the above assignments without a drop you have 17/42 = 40.5%. With dropping 1 lowest score, the score used will be 17/37 (dropping #4) or 17/32 (dropping #3) whichever results in the higher grade for the student. So in this case 17/37 = 45.95% or 17/32 = 53.1% are compared and the 17/32 option is the grade given to the student so #3 is dropped. In your case with 2 0's and dropping the lowest, it should always be the assignment worth the most points that is dropped.
Does that correspond with what you are seeing?
Ron