Credit card joint account

Im trying to buy a house and i was told i need to pay off some medical bills and get some revolving credit before i can get approved. I know there used to be something called Piggybacking where an authorized user inherited the original users credit account. I know FICO no longer calculates these accounts, but what if someone were added as a joint user where they were both accountable for any defaults? What would be the downfall by doing this? Does the good credit holder inherit the not so good credit holders history or is the one good account only transfered to the newely added joint user? What would the downfalls be by getting added to someones good account as a joint user? This would be a father son basis..............................


thanks for any replies