Per Form 8962 Instructions, Premium Tax Credit, 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.

The amount of the allocation is determined between those on the policy in any manner they agree on. The split can be any combination that equals 100%. It can be 100% to one and 0% to another, or any combination thereof. 

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

In the TaxAct Program, to enter shared allocation:

1. From within your TaxAct return (Online or Desktop), click Federal. On smaller devices, click the menu icon in the upper left-hand corner, then select Federal
2. Click on Health Insurance in the Federal Quick Q&A Topics
3. Click on Premium tax credit
4. Click continue until you reach the screen Health Insurance - PTC - Form 1095-A, then select Add to create a new form or Review to review already created.
5. On the screen Health Insurance - PTC - Allocation of Policy Amounts select Yes
6. Enter the appropriate information