You would have a coinsurance issue for being underinsured. Simply put, if you insure a 100,000 house for 50,000, the coinsurance ratio is 50% and only 50% of your claim would be paid. It's more complicated than that. You can read it in the Loss Settlement section of your homeowners policy.
It's more of a two-family dwelling than a duplex.
Ideally, you should have one policy on the dwelling with both of you as Named Insureds. Then it's just a matter of both of you contributing to the premium.
If you can't get him to contribute and you feel it's an insurmountable issue you can file a Partition Action in court and force the sale of the property so you can be done with him and get your own place.