test.php

<?php
$product = $this->Products->find()
    ->where(['Products.user_id IN' => [1, 2]])
    ->order([
        'GREATEST(COALESCE(Projects.created, 0), COALESCE(Projects.modified, 0))' => 'DESC',
    ])
    ->limit(10)
    ->offset(0);

debug($product->sql());

test.sql

SELECT FIELD1, FIELD2, FIELD3 
FROM products Products 
WHERE (Products.user_id in (:c0,:c1) AND (Products.deleted) IS NULL) 
ORDER BY GREATEST(COALESCE(Projects.created, 0), COALESCE(Projects.modified, 0)) DESC 
LIMIT 10 OFFSET 0