TWO INSURANCES

This is just a "mock situation." If I'm in the billing department of a DME supplier, and a patient has two insurances, a deductible and a coinsurance on one, what's what? If an item costs $2,000.00, and there's $100.00 left on a $500.00 deductible, will the coinsurance "kick in" for the remaining $1,900.00? left on the charge? Also, just saying that the percentage is 80/20 - the patient responsibility being the 20% of the $1,900.00, will the other insurance "pick up" the 80%? Hope this makes sense ...