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:
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.