"cannot emphasize more" is correct and can be used in written English.
Example: I cannot emphasize more how important it is to be prepared for your upcoming exam.
The importance of proper graphene transfer process cannot be emphasized more because it is so closely related to the performance and stability of graphene devices.
I emphasize more on the legs, more on the core.
"I cannot emphasize that enough," he says.
I cannot emphasize this point strongly enough.
I cannot emphasize enough how essential a read this is".
I cannot emphasize enough how helpful and polite everyone I dealt with was.
I believe we cannot emphasize bluntly enough that the cure and vaccine development depend on money.
"I cannot emphasize strongly enough that everyone should stay where they are.
I know I'm repeating myself, but I cannot emphasize enough how important hard work is.
But I cannot emphasize how totally beside the point all that is.
"I cannot emphasize enough how delighted I was to find that she is nothing like that.
