Example. A, who is married and files a joint return, receives $100,000 in compensation from her employer for the calendar year. B, A's spouse, receives $300,000 in compensation from his employer for the same calendar year. A's compensation is not in excess of $200,000, so A's employer does not withhold Additional Medicare Tax. B's employer is required to collect Additional Medicare Tax only with respect to compensation it pays to B that is in excess of the $200,000 threshold (that is, $100,000) for the calendar year.
Example. C, who is married and files a joint return, receives $190,000 in compensation from her employer for the calendar year. D, C's spouse, receives $150,000 in compensation from his employer for the same calendar year. Neither C's nor D's compensation is in excess of $200,000, so neither C's nor D's employers are required to withhold Additional Medicare Tax. C and D are liable to pay Additional Medicare Tax on $90,000 ($340,000 minus the $250,000 threshold for a joint return).
26 C.F.R. §31.3202-1