I've had a second passport for a while now. It's certainly possible to get one, but it's not guaranteed for anyone. The State Dept. approves them on a case-by-case basis.
It's not intended to be a "spare" passport. If you're applying for that reason, your application will most likely be denied. Come up with a better reason or make something up. The most common reason used is that the person travels often to places that require visas in advance, meaning that your passport spends a fair amount of time in the mail or otherwise not in your possession.
The good news is that it doesn't have to be restricted. The second passport that I have has no restrictions on it at all. It's just as valid as my primary passport. The only difference between the two is that the second passport has a valid period of only two years, so it needs to be renewed more often... and yes, when you renew it, you have to "re-justify" it.
https://my.usembassy.gov/u-s-citizen-se ... -passport/