Example. H, who is married and files a joint return, receives $100,000 in wages from his employer for the calendar year. I, H's spouse, receives $300,000 in wages from her employer for the same calendar year. H's wages are not in excess of $200,000, so H's employer does not withhold Additional Medicare Tax. I's employer is required to collect Additional Medicare Tax only with respect to wages it pays which are in excess of the $200,000 threshold (that is, $100,000) for the calendar year.
Example. J, who is married and files a joint return, receives $190,000 in wages from his employer for the calendar year. K, J's spouse, receives $150,000 in wages from her employer for the same calendar year. Neither J's nor K's wages are in excess of $200,000, so neither J's nor K's employers are required to withhold Additional Medicare Tax. J and K 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. §31.3102-4