You guys have always been very helpful; especially when all else fails. So first of all, I want to say thank you.

My question is, I have a letter addressed to a person, and I would like to send that same letter to someone else for reference. I would like both parties to know that this letter is being sent out. What is the best way to indicate that on the letter ?

Thanks again.
You put cc 'other person's name' on the letter. cc is short for carbon copy - from the days of typewriters - but it is still used to indicate a copy going to another party.