I agree with Thomas that this is not really a coding question, but rather a legal or finance question.
I imagine this is being done if the provider is credentialed differently with the primary and secondary insurance. For example, credentialed under Medicare as a group, but individual credentialing for United HealthCare. I don't believe it would be potential fraud unless they are billing the same service to the same insurance under multiple bills. The intention is to seek the balance after primary insurance, not to receive a duplicate payment.
I have seen similar situations, and I assumed the legal department, compliance department, or others with expertise in the area had reviewed this before instructing dozens of their practices to do so.
I will note there were some 2ndary carriers who had an issue with the primary EOB TID not matching what we submitted to them, and then had to appeal.