IDoc communication with transactional RFC (tRFC) uses all available dialog processes on the application server that transfers the IDocs to the communication layer. With tRFC parallel communication, it does not matter whether IDocs are first collected or transferred immediately.
IDocs should be transferred to the communication layer on an application server with a sufficient number of dialog processes.
Make sure you specify a sufficient number of dialog processes on the receiving SAP system. The number of dialog processes should not be less than the number on the sending application server.