What it sounds like you want to do is iterate through your list of lists, and at each list, randomly select an index, multiply the value at that index by 0.5 and place it back in the list.
import random
l = [[-0.03680804604507722, 0.022112919584121357], [0.05806232738548797, -0.004015137642131433]]
# for each sub list in the list
for sub_l in l:
# select a random integer between 0, and the number of elements in the sub list
rand_index = random.randrange(len(sub_l))
# and then multiply the value at that index by 0.5
# and store back in sub list
sub_l[rand_index] = sub_l[rand_index] * 0.5