Du kan använda en självanslutning:
INSERT INTO jobs
(customer_id, client_id)
SELECT customer.id, client.id
FROM users customer
JOIN users client ON customer.username = ? AND client.username = ?
Eller så kan du använda underfrågor:
INSERT INTO jobs
(customer_id, client_id)
VALUES (
(SELECT id FROM users WHERE username = ?),
(SELECT id FROM users WHERE username = ?)
)