Save data into 2 tables cakephp 3

I am very new to Cakephp 3. I try to do saveAll function in Cakephp 3, but unsuccessful.

I have 2 tables : Merchants and Users I want the create a merchant's registration form that will insert the data to both tables.

Can someone help me how to insert data into 2 different tables from 1 form using Cakephp 3?

Your helps is very appreciated. Thanks.