Insertar SQL en seleccionar de múltiples campos

Podría usar una autounión:

INSERT INTO jobs
  (customer_id, client_id)
SELECT customer.id, client.id
FROM   users customer
  JOIN users client ON customer.username = ? AND client.username = ?

O bien, podría utilizar subconsultas:

INSERT INTO jobs
  (customer_id, client_id)
VALUES (
  (SELECT id FROM users WHERE username = ?),
  (SELECT id FROM users WHERE username = ?)
)