Let me clarify my personal experience, and I only have 1 passport, from
South Africa. Now as most of you probably know, we are considered some of the least desirable people in the world and virtually every country put all sorts of hurdles in front of us that have to be overcome in order to enter the other countries. It is probably not helped by the fact that we had a national
service system in
South Africa and many conscript got special military
training like combat engineers with land mine, explosives and demolition
training.
Today, when you fill out a visa application, many countries ask questions about whether one served in combat or had specialized training and usually an entirely different form is available to be filled out describing your training, and especially if you were involved in combat.
The nett result is that one is thereafter profiled, I think the data is made available to interpol. So in 2000 I moved from the companies manufacturing operation in South
Africa to the engineering HQ in
Germany. I was assigned to lead the Ford Focus program, which had been launched but was still in production for several years. The job required that I get on a plane to London every 2 weeks for a meeting with the client at their Dunton Engineering center in London. I had a residence visa in
Germany since I was married to a german girl, but of course carried a ZA passport. I had submitted to extensive questioning prior to getting my residence
permit in Germany and separately for a tourist visa to the US which surprisingly was issued for 10 years.
On every trip to London (every 2 weeks) I would leave Germany without any fuss, then stand in the "non EU citizen" line on arriving at the
airport in London. Usually I had with me a printed copy of the meeting invitation, my
hotel reservation and of course my return plane ticket. Normally I would depart the UK later the same day, in the evening. Regardless, I was usually interrogated for 10-30min before being released by immigration. Going back the other way, the main problem in Germany was the length of the "non eu citizen" line in Germany, the border control people rarely said anything to me at all.
Then 911 happened, 1 year in to this job. Procedures changed quite a bit and the processing time was the main thing that changed when arriving at London. By then I had dozens of stamps in and out the UK in my passport and the folks at immigration were actually nice to try to get as many stamps as they could on a page before starting to use another page. This was a common problem with your passport pages getting filled up and requiring a trip back to South
Africa to apply for a new passport since it took so long to get a replacement through a consulate. All at your own expense, no refund from the company.
Then I started interacting with people from the US engineering center and that is where things changed pretty drastically. Because the 911 terrorists had entered the US through Frankfurt, special
security procedures were adopted at Frankfurt. One had an extra
security check prior to entering the secured area at the airport and then another completely independent security check at the gate where one would board. All I can say is that I was "randomly selected" for enhanced screening every time I arrived at that gate. Every item in my bag was unpacked and rifled through, I got a full pat down. Every entry in my passport was reviewed. I had to show the meeting invitation with a list of names of people I was going to meet.
Phone calls were placed to validate the info I had provided. In the meantime I was kept in a locked room.
Years later, I had moved to the US and was working under a
work visa. I still had to make trips to Germany, except I now lived in the US. Departing the US was no problem, yet getting cleared to board the flight in Frankfurt was still a nightmare. There were times I wondered if I was going to live out my life in a little room on the airport in Frankfurt if they decided I couldn't fly back to the US.... I was routinely asked by security officials if I was carrying any thermo nuclear devices, which initially I thought was a joke. They had
dogs sniffing all electronic devices.
Years more later, when I had my interview with the FBI as part of the background check for my green card application, I was surprised to find that the FBI agent was well aware of the situation regarding conscription in the South African defense force as well as the nature and extent of hostilities that we had been involved in. While my green card application was approved, I have regardless had never ending issues with the Virginia office of the FBI who handle background checks for firearm transactions and in 10 years have so far not had them clear any of my background checks without lengthy delays or requiring intervention by a supervisor. In some cases they have refused to clear a 4473 even after 3 months (and in contravention of the law) until a supervisor got involved.
So, if you intend to travel somewhere and have multiple passports, you will do well to clarify how their border control agents will react to that status. I'm pretty sure the minimum requirement would be to declare having multiple passports and then possibly choosing which passport would be used for the sake of entering and exiting the country. If traveling to authoritarian countries like
China,
Russia, Africa, the middle east, I would check very carefully before simply showing up with 2 passports. In a lot of these places its difficult enough for people to get 1 passport and 2 or more is the mark of a foreign spy.