Hi!
As mentioned by Evgeniy, all ME Sites should be located within the same SAP ME instance. It is due to the requirement that SAP ME and MII must co-locate on the same NW instance. Respective ERP plants can be located in different ERP systems indeed.
Just because of that requirement, SAPMEINT is implemented in the way that it uses API-calls to SAP ME that is possible only when SAP ME and MII are located on the same NW instance.
However, it is possible to switch SAPMEINT to using web services instead of API-calls. Then the information from Supported SAP ME Plants about servers can be used to address SAPMEINT requests to SAP ME instances located on different servers. But this is an obsolete configuration, not officially supported at the moment. It is likely you will have to engage SAP ME Consulting team if you need it to work this way.
Regards,
Sergiy