By allocating the amounts on Form 1095-A, you are not increasing your tax family size. That will remain at two. The only way to increase your tax family is to add your second child as a dependent, which of course means that your partner would not be able to claim that child. You are allocating a portion of the advanced tax credit received to your other child that will be reported on your partner's return.
You want to enter Form 1095-A exactly as it is reported to you for Boxes 1, 2, and Part III. Once you are done entering Form 1095-A and hit Continue, that is when TurboTax asks Let us know if these situations apply to you. One of the options is I shared this policy with another taxpayer who's not on my taxes.
This will allocate the premium tax credit you have received between your return and your partner's return. It will not change the calculation based on your household size, because that will remain at two. It should still reduce the amount of advanced premium tax credit you have to repay, but it is a completely different way to go about doing it than by increasing your household size. Again, the only way to increase your household size is by claiming your second child.
