List1 = ["a", "b"]
List2 = ["c", "d"]

# if you want the concatenate the two strings
List3 = [i+j for i in List1 for j in List2]

# if you want to make tuples out of the objects
List4 = [(i, j) for i in List1 for j in List2]

# the itertools module has functions for a lot of combinatorics operations
import itertools
List5 = list(itertools.product(List1, List2))

print(List3)
print(List4)
print(List5)