Wat? Of course front and back should be in same codebase. Why on earth would you need them to be in different repos? Put them in different folders if it makes you happy but there's no need to toss stuff all over the place. They're friends. They need to talk to each other. Let them live together
it's not that conclusive, there are many other aspects in which it can be bad (security, team's workflow etc). There is however no design principle ever dictates how components should/should not be stored in the same code repo.