How faculty might assess a PhD applicant who already has a PhD degree? What concerns/worries a potential PhD supervisor might have when he/she sees an application from someone who already has a PhD and applying for the second one? The fields of study are distantly related (engineering - physics).

What questions should be addressed in the initial letter by the applicant?

The first PhD is not a complete failure but an applicant wants to pursue his ultimate interests. The applicant has a postdoc position secured but has serious doubts working in the field that is not of major interest for him. Postdoc applications in the field of main interest didn't result in getting a position.
Here are the concerns I would have:


Is this person seriously going to spend another N-years getting a second doctoral degree, and then go seek a postdoctoral position before pursuing the job they presumably want to have? If so, what is this going to do to their career, and do I want to be party to that?
How did this person get all the way through a PhD in something other than their "ultimate interest"? Do they know what their "ultimate interest" is now?
Did they try to switch fields mid-PhD, or get some credentializing in the field they wanted to be in. If so, how much? If not, why not?
Do I think this person has strong prospects in this field? They've failed to get a postdoc - was this the result of just a small number of applications, or genuinely a problem. Again, if their postdoc application was weak, do I think it could be salvaged?