I need to write to two people at two different addresses, not one with a copy to the second. Do I stack them, or write them side by side?
Not sure what you mean.
Two people, different addresses = two separate letters, each with one person's name and address on it. If you want to indicate to each person that the same letter has been sent to the other person, then you put "cc xxxx" at the end of the letter after the signature.
Is there a particular reason for not doing this?