Step 1: Express the repeating decimals as fractions:
\[
0.\overline{3} = \frac{1}{3}, \quad 0.\overline{4} = \frac{4}{9}
\]
Step 2: Add the two fractions:
\[
\frac{1}{3} + \frac{4}{9} = \frac{3}{9} + \frac{4}{9} = \frac{7}{9}
\]
Thus, the simplest form of the sum is \( \boxed{\frac{7}{9}} \).