It can make sense to do this - because the grades a Uni wants in an offer may not be what they accept in August. If you think one of these Unis is more likely to accept you with lower grades, and these really are the only two Unis you want to consider, then its worth the risk. But you do need to have a Plan B in case neither accepts you - resets, Clearing strategy etc.
Btw, Cardiff is worth avoiding at the moment, Big staff redundancies, axed courses, and planned strikes. Whatever that Uni tries to tell you, it will have an impact on you as a student.