Do brake pads designed for both carbon and Alloy rims exist?
Unfortunately not. Carbon pads would work on alloy rims but not effectively so you might end up in a pickle if having to stop quickly. If you tried to use alloy pads on a carbon rim then you'd have the same performance issues with the added effect of squealing like hell and leaving serious scores on your rims due to the material hardness.

Since its so easy to change out pads, i'd just keep some spares and swap out at the same time as your wheels.