Forefront TMG HTTPS to HTTPS bridging configuration,how to select a certificate file to authenticate to the SSL Web server?

Hi all

My environment is Forefront TMG 2010+ SharePoint 2010.

In this document:http://technet.microsoft.com/en-us/library/cc441474.aspx

HTTPS to HTTPS bridging: 

Note:
This scenario requires a server certificate on the Forefront TMG computer in order to authenticate it to the external client and requires a server certificate on the backend Web server in order to authenticate it to the Forefront TMG computer.

These two certificate should be same, or not?

when i select as following pic, I can not see my certificate file. but i have imported it to this path: Certificates(locate computer)-> Personal->Certificates. and in the HTTPS Listener's properties can see it.

 Anybody have idea about this?