I just found a workaround, if all methods are in all servers, the calls work fine.

This is the new program for the COM server:

DEFINE CLASS ComServer1 AS MySession OLEPUBLIC

	PROCEDURE ReturnHello1() AS String
	RETURN "Hello-1-1"

ENDDEFINE

DEFINE CLASS ComServer2 AS MySession OLEPUBLIC

	PROCEDURE ReturnHello1() AS String
		RETURN "Hello-2-1"
	PROCEDURE ReturnHello2() AS String
		RETURN "Hello-2-2"

ENDDEFINE

DEFINE CLASS MySession AS Session 
	
	PROCEDURE ReturnHello1() AS String
	EndPRoc
	PROCEDURE ReturnHello2() AS String
	EndPRoc
	
EndDefine