Do you need several PhD’s to lecture in different fields (cognitive neuroscience and psychology)? Usually?
Note also that "neuroscience" is a very new term, dating to first use in the 1960s and common use only by the 1970s, which is also when the first departments with "neuroscience" in the name were created. I'm not sure when "cognitive neuroscience" was first used, but I am quite confident that the number of people with a "PhD degree in cognitive neuroscience" is very, very small. The number of people with a PhD degree called "neuroscience" before the 1990s is very small, much smaller than the number of people qualified to lecture in a neuroscience course who earned their PhD before then.
Typically the requirement to lecture in a course is that the university department that has some level of ownership/responsibility for that course assigns that course to be taught by that person. Hopefully they make this choice by choosing someone they feel is qualified.
There may sometimes be obstacles to certain people teaching certain courses according to the department they are part of. For example, if a cognitive neuroscience course is situated in a psychology department, then it is possible that only people with appointments in the psychology department will be chosen to teach it. However, the psychology department can hire whoever they want and their degree can be in any field; these are administrative obstacles.