How does an umbrella actually pay out?
The underlying policy handles the claim first and pays up to its limit. If the claim exceeds that, the limit is exhausted and the umbrella pays above it. It is a second tank rather than a bigger one, and it does not change your underlying deductible.
Do I report a claim to both insurers?
Report it to us and we will make sure both are notified. The underlying carrier manages it, and the umbrella insurer needs to know it exists, particularly if the demand is anywhere near the underlying limit. Late notice to the umbrella is one of the few genuine ways to damage the cover.
What if my underlying limit is lower than the umbrella requires?
Then you are responsible for the difference, because the umbrella attaches at a stated figure and will not drop to meet a lower one. It is a common and invisible fault, usually created by switching a policy without checking the floor.