20 C.F.R. § 404.441
Where both the insured individual and another person entitled (or deemed entitled) on the same earnings record have excess earnings (as described in § 404.430 ), their excess earnings are charged, and their partial monthly benefit is apportioned, as follows:
Example: M and his wife are initially entitled to combined total benefits of $264 per month based on M's old-age insurance benefit of $176. For the taxable year in question, M's excess earnings were $1,599 and his wife's excess earnings were $265. Both were under age 65. M had wages of more than $340 in all months of the year except February, while his wife had wages of more than $340 in all months of the year. After M's excess earnings have been charged to the appropriate months (all months through July except February), there remains a partial benefit payment for August of $249, which is allocated to M and his wife in the ratio that the original benefit of each bears to the sum of their original benefits: $166 and $83. His wife's excess earnings are charged against her full benefit for February ($88), her partial benefit for August ($83), her full benefit for September, and from $6 of her October benefit, leaving an $82 benefit payable to her for that month.
20 C.F.R. §404.441