If you are filing a joint return, there is no need to mark the shared policy box. You only need to mark the box and enter percentages if you are sharing the policy with someone NOT on your tax return. You can go back and uncheck the shared policy box and skip this if you are filing a joint return with your wife.
If you share a policy with someone outside your tax family, you must enter the shared percentages. You and the other party can decide the percentages.
Allocating policy amounts - IRS, Form 8962