Originally Posted by
bocastephen
I regularly travel with two passports - I use the US passport to leave/re-enter the US and the Canadian passport to enter/leave the countries on my itinerary, sometimes sprinkling in the US passport depending on the destination or what stamps I want to collect. This almost never corresponds to the passport that is inputed to my PNR and transmitted to the next country (except for the US passport re-entering the US or exiting the US).
Same way for me with my US/EU passports, except when EU gives me a free visa. When I went to Brazil, my US passport was kept in my pocket through the entire check-in process.