Yes - right after we got into our rental car, we cancelled the second. Both of our reservations were similarly priced so we just hopped on the first shuttle that came... that's how we chose which to try first! The line at the rental car place (was not long when we got there) was painfully slow but we were just happy everything went smoothly without an issue.
If you can get pre-cleared before flying into Hawaii, you can pass that line in the airport and get to the rental car agency faster.
Have a fantastic trip!!