Comparing values in separate lists

[ThiefOfTime]

I would suggest to use the zip function in combination with a for loop if you want to compare the first elements, then the second elements, etc.
so instead of you two for loops use:

list_1 = [12.4, 2.7, 5.1]
list_2 = [10.9, 2.3, 3.7]
for element_1, element_2 in zip(list_1, list_2):
       if element_1>=5 and element_2>=5:
            a.extend((element_1,element_2))
        elif element_1<5 and element_2>=5:
            b.extend((element_1,element_2))
        elif element_1>=5 and element_2<5:
            c.extend((element_1,element_2))
        elif element_1<5 and element_2<5:
            d.extend((element_1,element_2))
print(a, b, c, d)