Ankit, you're just looking at it in a wrong way. There is technically no relationship whatsoever between the inbound and outbound IDoc. You can use an IDoc to create master data in SAP. And you can also use IDoc to distribute master data from SAP (using change pointers in this case). These are two independent processes and you can use one or the other or both. At least I don't see anything that would prevent you from using both (have not done this on practice).
The configuration is completely independent, these are just two separate interfaces. I believe that's what the mystery gentleman meant. Other than configuring these two interfaces nothing special is required.
Your inbound interface will create a customer record. This should trigger change document creation (this process is the same in SAP regardless of how the record was entered). And, if you done the configuration right then it will trigger the change pointers and then the outbound IDoc.
It is not "outbound IDoc triggered by inbound IDoc", it's master data created by inbound and then distributed by outbound.
NACE has nothing to do with your interfaces - you're dealing with master data here, not transactional data.