Each other/one another

[neeti2711]

After many years of feeling separated by a great divide, the doctor now finds himself besides the lawyer, working with one another against HMOs and big tobacco

(A) besides the lawyer, working with one another
(B) besides the lawyer, working with each other
(C) beside the lawyer, working with each other
(D) beside the lawyer, working with her
(E) beside the lawyer, working with one another

OA: D

Please explain the use of 'each other' and 'one another'

[DavidG@VeritasPrep]

After many years of feeling separated by a great divide, the doctor now finds himself besides the lawyer, working with one another against HMOs and big tobacco

(A) besides the lawyer, working with one another
(B) besides the lawyer, working with each other
(C) beside the lawyer, working with each other
(D) beside the lawyer, working with her
(E) beside the lawyer, working with one another

OA: D

Please explain the use of 'each other' and 'one another'

There's some debate about this in my usage guides, but the general consensus is as follows -

Each other: a reciprocal pronoun involving two people. Tim and Tom decided to work with each other.
One another: a reciprocal pronoun involving more than two people. Tim, Tom, and Mary decided to work with one another.

In this case, because the subject is singular - the doctor - neither construction is appropriate.