If you and your spouse/life-partner have a joint checking/savings account, and bought a house/car with a joint loan, how closely tied are your credit scores? For example, if one person suddenly took on a lot of debt or ruined his or her credit score, does that affect the other person's score in anyway? How would it affect future joint loan applications?
Your credit score is only affected by the status of loans and credit accounts which are yours, either alone or jointly with someone else.
Bank accounts are not usually considered on credit reports or credit scores.
So to answer your questions:
If one person took on a lot of debt or ruined his or her credit score, does that affect the other person's score in any way? One person cannot directly affect another person's credit, except to the extent that they can affect (by not paying) shared obligations. If someone with whom you share a joint loan goes bad on other debts, it will affect their credit score but not yours.
How would it affect future joint loan applications? I'm not so sure on this one, as it would be up to the lender. They would probably consider both applicant's credit scores and reports, so one person's bad credit would probably affect the outcome of the loan application.
Your credit score will only be affected by accounts that you share.
So if you go bankrupt, and share no credit lines, mortgages, etc, your spouse's credit will not suffer at all. But, if you go bankrupt and discharge debt on a joint account, your spouse's credit will be affected negatively.
If one spouse has bad credit, the main consequence is that the only the income of the spouse will be considered by the underwriter. So if you make $200,000 (but have awful credit) and your spouse makes $75,000, your spouse will not have sufficient income for a $500k house.