"The rules state that the combined earned income of both parties can be used to calculate the maximum contribution and it can then go into one or other or both IRAs."
No, that's not what the tax code says. Only the spouse with the lower compensation is permitted to make a spousal contribution supported by the the other spouse's compensation. That's explicit in tax code section 219(c)(2)(B). The spouse with the higher compensation is not permitted to use any of the compensation of the lower-compensated spouse to support the higher-compensated spouse's contribution. Also, to make a spousal contribution the spouses must file jointly.
TurboTax calculates the permissible contribution amounts correctly.
