I second what ilecs says and would point out that its not wise to piggyback on a mortgage because if will severely affect your own dti when it comes to buying a place. You might be able to piggyback on a home equity loan or line of credit but again I'm not sure if this would be feasible or smart. Best to only piggyback on loans if you 1) are prepared to pay the loan or payment if the primary account holder fails to pay and 2) deal with negative consequences of someone else's actions such as a missed payment or high utilization.... granted authorized User accounts are easy to remove; but if the account is a joint account then there is no way of getting off the hook if things go south with the account.
FICO 9: 
Filed Chapter 13 on 6/1/2017 after job loss. Discharged 6/1/2022.
Goal: Gardening!
Filed Chapter 13 on 6/1/2017 after job loss. Discharged 6/1/2022.
Goal: Gardening!
An AU is an authorized User of a credit card account. Some credit card accounts report the payment status of the account to the authorized users credit file. In this way the autborized User can benefit from the credit history of the account especially where the history is long and clean. My husband is an AU on my American express; chase; and discover credit card accounts... as well as a joint account holder for our new vehicle loan. In this way he is piggybacking from the old history and also he contributes to spending for earning rewards. Mortgages cost thousands to refinance. Its a better idea to use those thousands to open a high limit secured card or loan in your own name to build individual payment history.
FICO 9:
Filed Chapter 13 on 6/1/2017 after job loss. Discharged 6/1/2022.
Goal: Gardening!
Filed Chapter 13 on 6/1/2017 after job loss. Discharged 6/1/2022.
Goal: Gardening!