The answer to that question depends on the rules of Coordination of Benefits. Sometimes Medicare will pay first, and your secondary insurance will pay second. Sometimes it is the other way around. What happens is that each type of insurance that you have is listed as a payer. Then each payer is assigned a number Primary payer, Secondary payer, etc. Who pays first is determined through a range of rules.
Things you can do to Help the Process
Give all of your insurance cards to your healthcare provider to copy. This helps your healthcare provide sort out who to send bills to first. You can call the Benefits Coordination & Recovery Center at 1-855-798-2627 toll free. Have your Medicare card handy when you call. This is a company that acts on behalf of Medicare to help answer questions about pay order for Medicare.