Do you use "to whom it may concern:" with "Dear Mr./Mrs" or only pick one?

Question

I've seen example where people use both, for example:

To Whom It May Concern:

Dear Mr.,

I am writing to apply...

OR is the correct usage this:

To Whom It May Concern:

I am writing to apply...

Secondly, should "to whom it may concern" also be put into the address portion of the cover letter? That is, in the first line above the address of the employer? For example:

To Whom It May Concern
RBC Bank
123 Bank St, Suite 1
City, State 13235

OR, should it just be:

RBC Bank
123 Bank St, Suite 1
City, State 13235

Thank you in advance

Answer 1

First, "To whom it may concern" is saying that you do not know the identity of the person you are writing to or you are writing the same letter to more than one person. So a 'Dear Mr.' after does not apply.

Second, Try your best to find the name of the employer; you could do this by calling the hiring agency. This is also unusual and might be a clue that the agency does not know what it is doing or is going to submit your application to various job positions. A generic letter does not look as good as one addressed specifically.
But if for some reason this does not work then include the name of the hiring agency through which you are applying.

I wish you the best.