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If having double citizens is against the law in one of the countries involved, it is better not to bring the other passport when visiting there.
Otherwise bring whatever passport makes more sense for the trip (visa-wise), both if necessary.
Also remember that you are required to always enter a country with that country's passport if you are citizen - not allowed to use the other one!

I have had 2 x passports for many years now. Always travel with both. Never had any problems.

better bring both, just to make sure

To be sure, if you are a US citizen, you must use your US passport to leave or enter the US.

Thirdcoastryan wrote:

To be sure, if you are a US citizen, you must use your US passport to leave or enter the US.

That's the easy one my question was more for trips like I just did, from Norway to Taiwan via Dubai, where I don't technically need both passports. At this point I've resigned to always having both just in case.

Thirdcoastryan wrote:

To be sure, if you are a US citizen, you must use your US passport to leave or enter the US.

And advice for those who are not US-citizens, when entering the US, make sure you go in with the same nationality-passports you used last time. My wife failed to observe that rule, and she gets put in the detention-room every time she visits, now.

Gordon Barlow wrote:

And advice for those who are not US-citizens, when entering the US, make sure you go in with the same nationality-passports you used last time. My wife failed to observe that rule, and she gets put in the detention-room every time she visits, now.

Gordon: Does it help if she shows both/all now? I know a US/Philippines dual citizen who was told to show both her US and Filipino passport when entering/departing the Philippines, but there was no explanation as to why (though I could guess that they want to make sure you're not somehow flagged under one passport when you aren't on another).

It's an interesting subject.  My kids both have two passports and wherever we go it's always very clear which one they'll need to use in the country we visit based on which works better for visa requirements. 

I'm thinking my wife would always bring the other as well but I'm not certain of that.  I'm pretty sure there has never been any case where we've showed both passports to anyone related to immigration processing.  Of course when entering the two countries they have passports for they always use the one for that country.

My young son is a dual (Canadian/Brazilian) he must use his Brazilian Passport to enter and exit Brazil and also must use only his Canadian Passport to enter and exit Canada. That's the way it works in most countries, so it would in most cases be absolutely necessary to carry both passports.

For entry into third countries, it would make sense to use the passport that is most acceptable in that country. Henley & Partners puts out an annual list ranking passports for acceptability:



Cheers,
James     °µÍø½ûÇø-blog Experts Team

James wrote:

My young son is a dual (Canadian/Brazilian) he must use his Brazilian Passport to enter and exit Brazil and also must use only his Canadian Passport to enter and exit Canada. That's the way it works in most countries, so it would in most cases be absolutely necessary to carry both passports.

If he travels from Brazil-Canada-Brazil, what is the passport he should use?

mal wrote:

If he travels from Brazil-Canada-Brazil, what is the passport he should use?

I´ve found this guide to be very useful when sorting out how to travel with two passports.