You can often share a renter's insurance policy with your flatmate. There is one caveat, however, and that is that everyone included in the policy must also be listed on the rental agreement. If you sublet or stay on your friend's couch, you cannot share her renter's insurance policy. If you share a policy and your roommate has a claim that does not affect you, that claim would be on your insurance record and could affect your insurance costs in the future.
If splitting the renter's policy will save you and your partner a good deal of money, go for it. Renters' insurance liability claims do not occur as frequently as other types, but they are some of the most expensive claims to file and companies take severity into account when assessing rate increases and non-renewals. As a policyholder, you don't want to face years of higher property insurance rates because you had to file a claim because of your roommate. One flatmate is not covered by another's renter's insurance because the policies are not adequately priced for that risk.
This means that he or she may have to pay more at renewal or if he or she changes insurer because he or she would look like a risky customer for a claim that he or she had nothing to do with. If a flatmate included in a renter's insurance policy has to make a claim, the consequences for the insured can be serious. If he or she files a claim and the resulting check from the insurer names both of them, he or she may have difficulty collecting. An older sibling should take out his or her own renter's insurance policy, separate from that of the sibling with whom he or she is staying.
Yes, there are a number of things you can and should do to protect yourself if you decide to split a renters' policy with someone. This is because your parents' homeowners or renters insurance policy may include coverage for your personal belongings if you live on campus. Roommates are excluded from all renters' insurance policy coverages, including personal property, liability, and loss of use in the event the rental becomes uninhabitable. However, roommates who rent an off-campus house or flat should each have their own renters' insurance policy.
Many insurance companies offer renters insurance to roommates, but there is no guarantee that sharing a policy will save you money in the long run.