Might be O2 sensor fault?
I would have thought that blocked cats would get very hot instead of not heating up?
More details might help.... error codes?
Hopefully it is just an O2 sensor error code & you might be able to get away with just replacing a pair of sensors.
I think he might have been talking about back O2 sensors rather than back cats
as they only have one cat on each side & sensors in front of and behind each cat.
(You using the word "back" in your post is another reason I think that)