You don't indicate one way or the other so I'm going to answer assuming the patient was seen by both providers on the same day and that this is a bundling denial. Some plans will only one one E/M per group per day so if that's the reason for the denial, none of this applies. If you're not sure, call BCBS for clarification of the reason for the denial.
First, verify the diagnosis codes attached to the claim to make sure they are not billing the same codes. The last practice I was at, we had a couple of oncologists who did pain management clinics a couple of times a week. We always made sure to report diagnosis codes for the pain and related symptoms before including the underlying condition. If they were seen by an oncologist the same day, any pain code was far down the list of ICD-10 codes.
Second, are you using a -25 modifier on the second visit of the day? Two E/Ms billed on the same day, even if for different providers, will bundle together.
If your codes are correct and you reported a modifier on the 2nd visit, you will need to submit an appeal to the payer and supply medical records to show the denied visit was separately identifiable from the other visit that day.