Why does turbo tax and mine and my husbands income together when calculating the additional medicare tax. Shouldn't it just be on the one that is over 300000.

Where are you getting the $300,000 number?

No, it is the combined amounts for a married filing jointly return. Form 8959 (Additional Medicare Tax) is used to figure the additional 0.9% percent Medicare tax on the portion of wages, self-employment, and railroad retirement income that exceeds a certain threshold ($200,000 for individuals, $250,000 for spouses filing jointly, and $125,000 for spouses filing separately).