How to configure 2 certs for one partner in TN

We have a partner who is using Microsoft Biztalk for all B2B transactions. They provided us 2 different certificates. One is from their outbound box inside the firewall, and the other is from their inbound box on the DMZ. What we can do is to install both certs in the file directories on RI/IS servers and admin pages as well. However, we have no idea on how to install both certs under the partner profile in TN. We tried to install either their outbound or inbound cert but none of them seems to work. Accoording to their consultants, they have successfully communicated with other webmethods partners via HTTPS before but they don’t know how those partners took care of 2 certs issue. Anyone have any ideas on how to deal with 2 differents certs?