The answer is 30kV.
My method led to an answer of 41KV, where did I go wrong?
I believe I cant just add the Vmax(s) at the end, but what would I do instead and why?
If C1 and C2 are in parallel then they always have the same voltage across them, and their maximum applied voltage is equal to the lower voltage rating of the two capacitors. You can't treat them as independent devices. Combine C1 and C2 into a single equivalent capacitor and see what you circuit looks like then.