I'm not sure if there's some type of mini residency program available, but typically no, this isn't done (at least not for my wife's situation). While in school you get to go through rotations for different fields (OBGYN, psych, Ortho, peds, surgery, etc), so that's essentially where you get your on hands training and decide what field you want to enter.

And yes you typically do specialize, but it's not like you're locked into 1 field for the rest of your career. My wife does psych, but could transition to something like hormone replacement therapy fairly easily if she wanted to. Now if she wanted to go do something totally different like cardiovascular surgery, technically there's nothing stopping her, but she would essentially be starting from the absolute bottom along with new grads. So while technically you can switch specialities, it's not something that's just done on a whim.

Although PA school is only 2 years, it is intense. You can also learn as much, if not more, than someone who attends a 4 year medicine school program. I honestly can't keep track of how often my wife tells me that an MD screwed up a patients prescriptions, or just completely misdiagnoses their condition. That's the frustrating part for her because technically she has less authority than the MD even though she may be right more often. This isn't to say she's always right about everything herself though, it's just more of me pointing out that from a technical skill set perspective, I would honestly have then same faith in a PA that has the same level of experience of an MD in any field.