I hope you have a better experience at the Namanga broder crossing than I did. When I was there several buses and minivans from NBO arrived at the same time and it was a complete madhouse inside the Tanzanian Immigration Control building. There was no queue structure, just dozens of people pushing toward the counter and thrusting their passports, visa applications and money towards the officials. After the passports were processed an official emerged from the back room and literally threw 15 or 20 passports at a time onto a table and the mob frantically went through them one by one. There was absolutely no order to it and nothing to prevent somebody from taking the wrong passport (or multiple passports for that matter) and just leaving. It was the most nerve-wracking border crossing experience I ever had.