I know this is right when it's long hours, but not sure on a 10am opening. I've been on many short hour days but just can't recall.
In any case if you are there before the secruity check opens, they will clear you out of the secured area between the parks, and you will have to get in line and go through the check point to come back in. It's no problem to go through that area before the security checks if you want to take pictures or need to get from Harbor to the resort hotels for breakfast.