Per IRS Instructions for Form 8962, Premium Tax Credit, on page 6:

Note. If an individual in your tax family was enrolled in a policy with an individual in another tax family and you are not taking the PTC, the taxpayer who is claiming the individual not in your tax family may agree to reconcile all APTC paid for the policy.

For more information, refer to the examples in the instructions for Form 8962.

To enter shared allocation:

1. From within your TaxAct return (Online or Desktop), click Federal. On smaller devices, click in the upper left-hand corner, then click Federal.
2. Click Health Insurance in the Federal Quick Q&A Topics menu to expand, then click Premium tax credit.
3. Continue with the interview process to enter all of the appropriate information.
4. On the screen titled Health Insurance - PTC - Form 1095-A, click + Add Form 1095-A to create a new copy of the form or click Edit to review a form already created.
5. On the screen titled Health Insurance - PTC - Allocation of Policy Amounts, select Yes, then click Continue.

Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.