"So does Carnival stop at the same island? I assumed that part of the island was branded for HAL and part of the island was branded for Carnival; which would not preclude both ships beign docked at the same time. When Carnivals docks there; do they change over all the signage from HAL to Carnival? Just curious."
No part of the island is not branded for HAL and part for Carnival.
Carnival ships only stop at HMC when HAL ships are not there -usually in the summer months. You might find the odd Carnival ship at HMC in the winter months but not when a HAL ship is there. Sometimes you will find 2 HAL ships at the island but definitely not a Carnival & HAL ship together.
HAL ships are either in Alaska, Europe or other parts of the world in summer.
I have no idea whether they change over all signage from HAL to Carnival.