Assume the network issued on the first neuron 0.7, the correct answer is 0.2 error is 0.5. The second neuron produced 0.1, the correct answer is 0.5, the error is -0.4.
If you do (loss1 + loss2) .backward () -> (0.5 - 0.4 = 0.1) .backward (), how does the network know that for the first neuron you need to decrease by 0.5, and for the second to increase by 0.4?