I have a form called Clients and a form called Policies, which has a lookup field "Client Name"

One client can have multiple policies.

Both forms have a "Inactive" decision box field as well as a "Inactive Date" field.

What I need is when the inactive box is checked, and the Inactive Date is filled in the policies form, to also check the inactive box and fill the inactive Date in the Clients form, but only if that client does not have another policy that is still active.

How would i do this?