Sir
In sa 620
What is the need of agreement between auditor and auditor expert? 
Whether there points will be considered wrong.. :
1) to undertake the responsibility of both the auditor and the expert so that any confusion or misunderstanding should not arise in future. 
2) to have legal enforcement with the available contract if any of the conditions are breached by any party. 
Etc.. 
Whether these points are not the need of having an agreement between two parties..?