Originally Posted by
mitchmu
"In former times (before the latest round of State Department policy reforms), attempting to enter the US on a foreign passport could even be used as "evidence" of intent to relinquish US citizenship. Although this apparently isn't a danger any more, the best thing to do is probably to make life simple and assert only your US citizenship rights when entering the US."
In the sense that you technically do have to announce that you're a US citizen if you are one and are entering the US, yes; CBP could make your life temporarily harder than it needs to be and you could get fined. After a series of Supreme Court cases that made the State Department change their policy, it's very difficult to lose US citizenship. You either have to formally renounce it at a US consulate, or join a foreign army and take up arms against the US.
Even if you take up citizenship of a foreign country and have to renounce all foreign citizenships as a part of their naturalization process, the US doesn't care and you remain a US citizen. Renunciation has to be done through the formal process according to US law.
I'm not an immigration lawyer, and if you do run into trouble it's worth consulting one since the law can get complex, but I wouldn't worry about inadvertently losing US citizenship.
Why do you have 2 US passports?
Probably one's an official US passport for US government business. You still have your normal "tourist" passport for personal travel.