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: