It is my understanding that a company is not allowed to have more than one CEO. If this is true, similar restrictions should apply to the CFO. Could anybody tell me whether it is legal (in Delaware) to have 2 people with the CTO job title, or there must be one at most?
You can have as many people with the same job title as you wish.
You can have more than one CEO as well, although that would be weird and confusing. Nothing prohibits this and CTO isn't even a traditional and core executive title anyway.
This will be confusing to everyone involved, as grammatically and in the common English language usage of the word, a C-level officer is usually singular, so the people with that title may resent the fact that they don't really hold the post that their title implies, but tough. By way of comparison, it isn't unusual for big banks to have hundreds of vice presidents.
Also, third parties are entitled to assume in a legally binding way that everyone with the title CTO has the authority associated with that title, even if that isn't the case, under the agency principle of "apparent authority". So, the more C-level officers you have, the easier it is for the company to be legally bound by actions they take and the harder it is to assert centralized control over company operations.