This document explains that due to Identity Manager's protection of TEWS all SIGMA servers must be moved together to integrate with SiteMinder.
Identity Manager enables SIGMA through its TEWS web services. Therefore upon integrating with SiteMinder all SIGMA servers must integrate. There can not be a situation where one SIGMA server is integrated with SiteMinder and the other isn't. It is because the underlying Identity Manager's TEWS requires the SiteMinder protection and therefore no SIGMA server can communicate with Identity Manager unless also protected by SiteMinder.
Any Identity Manager version.
Any SIGMA version.
Any SiteMinder version.