I carry very good medical/dental insurance through my employer. My ex wife has shared custody with my daughter. My son, who is over 18 is also covered under my insurance. My ex wife’s new husband signed both my daughter and my son up to obamacare as a “secondary” insurance. Is this legal? As a tax payer it concerns me that obamacare can be used as a secondary insurance, when insurance is already being provided.
You can get ObamaCare as a secondary insurance, but you can't get tax credits on it. Generally holding more than one comprehensive insurance plan makes no sense. However, there are exceptions to this rule (such as when both spouses have an employer plan, when a person holds Medicaid and Medicare, or when a person holds multiple parts of Medicare).
If your dependents have coverage through your employer, then they can't get cost assistance through the Marketplace. So technically they shouldn't be getting tax credits, that said the person filing for the children is responsible for ensuring they are covered. The correct thing to do would be for you to provide them coverage through work, or for them to be on the Marketplace plan without cost assistance (if it is going to be used as secondary insurance).
Also, this doesn't override divorce agreements it just goes hand-in-hand with it.
We have provided additional information on cost assistance and coverage in a divorce or separation here.