This was something that was not available for a long time but with the new fix pack. This is available. I might have an old blog where i had mentioned, it was not available in the previous versions.
Many WebSphere Commerce websites are using REST services to serve both mobile and desktop pages then it's important to allow multiple logon for the same user from different channels in a simple way. To activate this feature a specific tag should be added to the wc-server.xml in the SessionManagement section
<allowmultiplelogonforsameuser display="false" enabled="true"></allowmultiplelogonforsameuser>