I would suggest signing one loan each, and particularly have her sign for the less-expensive of the two cars because there will likely be a higher interest rate if she doesn't have you on the loan. The size of the loan has less impact on her credit than a history of timely payments.
If you are planning to be married for more than seven years, that should be enough time to repair her credit with nothing more drastic than using credit wisely.
If you think the marriage may end before seven years, then cosigning on car loans can add complexity and lawyer fees to untangle when you go your separate ways.
In either case, staying together or separating, cosigning will add little value other than saving some interest. You may improve your finances more by spending less on the car, which the higher interest rate--from having the loan in her name--adds incentive to do.
The best way for you to help with her credit is to embrace good budgeting and spending habits together. Pay off unnecessary debt, and hang on to these cars instead of trading them in when the loans are paid.