The only thing that "joins" the credit of a married couple is the ownership of joint accounts. In other words, if there is a credit card, line of credit, mortgage or loan in both names or one party is an authorized user on one of the other person's account, then the credit report agencies include this information on the credit reports of the husband and the wife. In the event that there is a default on any joint account, that information is also reported on both credit reports.
Filing a joint tax return does not link the credit of a married couple. It increases the eligibility and amount of certain credits. The only connection between a joint return and a couple's credit is if there are back taxes or child support owed, default on a loan or tax balance due. If a couple files jointly and one or both of them owes child support, back taxes and has a default against a loan, then the money owed is deducted from any refund due. If the tax return shows that instead of receiving a refund, the couple owes money, then the IRS holds both parties responsible. So, filing jointly does not have the effect of "joining" the credit of a couple. Rather, a joint tax filing can affect a joint tax return.
To learn more, check out The Importance Of Your Credit Rating.
This question was answered by Chizoba Morah.