To find the sum of the populations of the two cities, we add them together:
- First city population: \(3.45 \times 10^6\)
- Second city population: \(1.3 \times 10^5\)
Since the two numbers are in different powers of ten, we need to express them with the same exponent before we can add them. We can convert \(1.3 \times 10^5\) to the same power of ten as \(3.45 \times 10^6\):
\[ 1.3 \times 10^5 = 0.013 \times 10^6 \]
Now we can add the two populations:
\[ 3.45 \times 10^6 + 0.013 \times 10^6 = (3.45 + 0.013) \times 10^6 = 3.463 \times 10^6 \]
So the sum of the populations of the two cities in scientific notation is:
\[ \boxed{3.463 \times 10^6} \]