If the primary key for a parent is assigned by the EIS, you can use a multilevel insert cascade operation to create the parent and child objects in a single operation without synchronizing multiple times. The returned primary key for the parent's create operation populates the children prior to their own creation.
The following example illustrates how to submit the parent MBO which also submits the child's operation:
Customer cust = Customer.findById(101);
Sales_order order = new Sales_order();
order.setId(1001);
order.setCustomer(cust);
order.setOrder_date(new Date());
order.setFin_code_id("r1");
order.setRegion("Eastern");
order.setSales_rep(101);
order.save(); // or order.create();
cust.save();
cust.submitPending();