I have 2 nationalities and 2 passports

[Comic 66]

I have 2 passports which allow me to travel to the US anytime I want but the K1 visa is in one of them, can I still travel I the US in the other passport without the visa until I'm ready to move up on the K1 visa?

[forrestc 2]

Again... different countries, different rules, but I know that they specifically told myself and my fiance (Canadian) that we could visit even with the K-1 in the passport - we just had to be VERY VERY clear we were not activiating the K-1 visa. At that point, it's just like any other visitor entry into the US - they have to be convinced you're coming to visit and you have to meet all of the other entry requirements.

Having not actually attempted this, I can't vouch for how it will go. In addition, as this is across the Canada-US border it might be a bit different.

I'm totally talking out of my area of knowledge in this next part, but I would suspect that the border agents might get suspicious if you tried to enter on a passport other than the one you have the K-1 in. To me it would seem like you're trying to hide something, which is never a good thing when you cross the border.