- Can I, as a dual citizen get a IEC Working Holiday Visa for Canada?
Yes.
- If I have to enter and leave the U.S. on my U.S. passport legally (born and live in the US), how would I say, switch to my Portuguese passport for the holiday visa, legally? I will be driving.
The US has no exit controls. When you drive into Canada, just show your Portuguese passport. (You may want to open it to the page with the visa to avoid confusion.) The border officer will probably ask you about your immigration status in the US, at which point you'll say you're a US citizen. If the officer asks to see your US passport, show it.
Canada also does not have exit controls. When you return to the US, just show your US passport. They probably won't ask you about your immigration status in Canada, but if they do, tell them and show your Portuguese passport if they ask for it.
Canada and the US have an agreement whereby they share information on land border crossings, so the entry into one country can be recorded as an exit from the other country. Therefore, when you enter Canada, you might want to ask the officer if you need to do anything special to have your exit recorded, in light of the fact that you will be using your US passport to enter the US after having entered Canada with your Portuguese passport.