Hello @ABT 

You need to make couple of setting in order to achieve what you want.

1. Create separate customer groups for each interco.
2. Replace existing customer group of interco customer with the new group.
3. You need to amend posting control setup for IP2. It needs to be based on control type customer group.
4. In posting control detail match interco customer groups with interco AR trade account and non-interco group with regular AR trade account
5. Once you complete the setting all new transactions will be posted interco AR trade
6. If you want to correct existing records the go to repost customer ledger item window. Find interco invoices and repost according to the current setting. 

Hope this helps

Perfect. Thank you