XDB and several instances on same machine ?

Hi all, I've got some doubts about XDB and a multi-instance config. I've got a standalone config where XDB/APEX works without troubles. Trying to make the config on a Windows machine where runs several instance (mono instances, not RAC), I'm getting troubles. It appears that one of those instance, let's say MAR has XDB running on port 8080 - if I try to start/config XDB on a second instance, let's say DWH on port 8081, the only XDB HTTP port that replies is 8080, not 8081. Is such a config supported or known to be cause of issues ? Thanks, frederic VDW (Technip).