Combining two integer columns without addition

Question

I have two columns:

Data = {\'Col1\': [1 , 2 , 3 , 0 , 0 , 4] , \'Col2\': [0 , 2 , 0 , 5 , 6 , 4]}
df = pd.DataFrame(data = Data)
df.head()

Col1    Col2
0