In reply to Marina.Miao:
When using get_next_item(), in order to merge your first item with a second; you’d need to call get_next_itme() a second time, before calling item_done() for the first item.
When using the driver pattern:
seq_item_port.get_next_item(req);
//...
//drive signals
//...
seq_item_port.item_done();
You must call item_done() before calling get_next_item() again, otherwise you’ll get an error.