Answer
The ratio is 2 to 1.
Work Step by Step
The voltage across $C_1$ is the 10 V from the battery.
The other two capacitors are identical and in series, so they each have 5 V across them (check: the total voltage across the pair is 10 V, as expected).
$$\frac{V_1}{V_2}=\frac{10V}{5V}=2$$