In addition to the normal rules for reconciling premium tax credits, the IRS also has an alternative calculation for the year in which you get married, which is described in IRS Publication 974. The alternative calculation allows you to reconcile subsidies differently for the months prior to the marriage. For those months, you can use half of the total married household income, and a household size that does not include the spouse (or the spouse’s dependents, if applicable).
This calculation is optional, so you can choose to use the normal subsidy reconciliation rules, which would mean using — for the entire year — your total combined household income and a household size that includes both spouses (and any dependents, if applicable).
So for example, say Bob and Casey get married in September, and we’ll assume neither of them had any dependents. Bob had Marketplace coverage prior to the marriage, but switched to Casey’s employer-sponsored plan in October. Bob’s income is $40,000 per year, and Casey’s is $70,000. When Bob reconciles his Marketplace subsidy for January – September, he can either use a household size of two and a household income of $110,000, or a household size of one and an income of $55,000 (half of their combined total household income).