Hi,
Allow me to help you analyze the language usage conundrum.
The sentence "Attached (is, are) the approval letter for your signature and the letter request from Company X" mentioned on your post is a passive construct in its inverted structure. To answer your question about verb agreement, let's revert its structure to its normal order.
THE approval letter for your signature and THE letter request (request letter) from Company X (is, are) attached.
Now that we have it in normal order, let's star analyzing the structure.
There are two subjects in the sentence presented above. These subjects are connected by "AND".
S1: THE approval letter for your signature
S2: THE letter request (request letter) from Company X.
Conj.: AND
So the subject structure calls for a plural verb (are).
To further analyze the structure, notice that I have capitalized the article "THE" in the sentence above. If my memory serves me right, the separate use of the article after the coordinating conjunction (AND) clearly indicates that the subjects are distinguished from each other and are therefore treated plural.
If you would like to avoid such problem, you may recast your sentence by making it in the active voice.
"I have attached the approval letter for your signature and the letter request from Company X."
But, of course, its appeal to the reader or the recipient of your letter will be different compared to the passive construct, which ,for me, sounds more formal.
Let's wait for sir Joe for further explanation or clarification if i missed some points on this post.
