Should I temporarily disable foreign key constraints? How?

I have two tables:

person:
    id serial primary key,
    name varchar(64) not null

task:
    tenant_id   integer not null references person (id) on delete cascade,
    customer_id integer not null references person (id) on delete restrict

(They have a lot more columns than that, but the rest aren't relevant to the question.)

The problem is, I want to cascade-delete a task when its tenant person is deleted. But when the tenant and the customer are the same person, the customer_id foreign key constraint will restrict deletion.

My question has two parts:

1. Is temporarily disabling the second foreign key my only option?
2. If so, then how do I do that in PostgreSQL?