The joint threshold is not twice the single one
Most thresholds in the tax code double for a joint return, or close to it. This one does not. The figure for a couple is meaningfully less than two single thresholds added together, which means two people can each be below their own and above the couple’s when their wages are combined.
Nothing about either salary looks unusual. Neither person would owe the surtax if they filed alone. The liability comes from the return rather than from the wages.
Nobody withholds it
This is the part that turns a rule into a bill. An employer withholds the surtax only once the wages it pays pass a flat threshold, and that flat threshold is higher than either spouse’s share. Neither employer withholds anything. Neither employer is wrong.
So the whole amount falls due at filing, in a year when nothing changed and nothing was flagged. It is not large as a proportion, being 0.9%, but it is unexpected, and it arrives alongside everything else due that April.
What you can do about it
There is no way to make an employer withhold it: the flat threshold is statutory and the employer has no view of a spouse’s pay. What you can do is cover it deliberately, either by asking for additional withholding on a Form W-4 or by making an estimated payment.
The figure is worth working out in advance for that reason rather than discovering it. The surtax calculator takes both sets of wages and shows what is owed and what, if anything, was withheld against it.
Filing separately is usually worse, not better
It is a natural thought, and for this particular tax it backfires: the threshold for filing separately is the lowest of all three, at half the joint figure. Splitting a return to avoid the couple’s threshold usually means meeting a lower one twice. There are sometimes good reasons to file separately, and this is not one of them.