Start with #1. Assume it is good. So try 2; if no light try then 3; if no light then try 4; if no light then try 5; if no light then try 6. If 1 was good then 2, 3, 4, and 5 must be bad and 6, 7, 8 good. If still no light after 6 then 1 must be bad and you know nothing about the rest. Set 1 aside and repeat starting with 2.
Starting with 1 it took 5 trials to either find two good ones or prove 1 was bad.
If 1 bad it will take 4 trials to find two that are good or prove 2 is bad.
So minimum number of trials is 5+4+3+2=14 trials to be sure that you find two good ones.