It is extremely difficult to get the FCC to sign off on CB/ham combos, which is why there aren't many. Amateur Radio is governed by Part 97 of the FCC rules, and CB is governed by Part 95, and they have different requirements.
HOWEVER...
Buying a multimode HF ham radio with a general coverage receiver CAN do both in an emergency.
My years-old Kenwood TS-50 has the "transmit unlock" modification done to it, so it can transmit out of band. I have all 40 CB channels programmed into the memory. I have tested it, and it works perfectly as a CB radio. Now, it would be contrary to FCC rules to run this as a CB radio. But during emergencies, anything goes as far as communications. So I am ready. In the meantime, I can legally LISTEN to CB channels on my ham radio, even if I can't legally TRANSMIT on them.
I hope you understand me here...