How is primary vs secondary insurance determined when claims are filed?

I'm 24 (25 in a couple of months), so under the ACA I'm still covered by my parents' insurance (insurance 1) for another year or so. I started a new job last year, so now I'm double-insured. When I reordered medications/supplies and went to the doctor, I made sure to add my new insurance (insurance 2) to my accounts as a secondary insurance.

I have type 1 diabetes and use a continuous glucose monitor (CGM). About a month ago, I got a letter from insurance 1 disputing a payment claim from November for the CGM, saying they had been charged and paid as the primary, but they were the secondary. I just got an EOB from insurance 2 from February for the CGM, and they paid absolutely zero, leaving me on the hook for over $6k. I don't have any EOBs from insurance 1 regarding this CGM order, so I'm wondering if the claim was even sent to them at all.

My question is, how is it determined which insurance is the primary vs the secondary? I assumed insurance 1 would be my primary for now since it's the one I've had the longest, but I wonder if it's no longer my primary since I'm covered as a dependent. Insurance 2 is held in my name.

Additional info:

Insurance 1 (parents' insurance):

Deductible: $700; $133.40 remaining OOP Max: $12,000; $9,806.03 remaining

Insurance 2 (my own insurance):

Deductible: $250; $0 remaining OOP Max: $2,000; $1,201.26 remaining submitted by /u/MiyaDoesThings
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