@Patti in Upstate NY I hope I did not scare you into dumping your clients. That was not my intent.
I think how I would handle this is just have a conversation with each, explain that you have a conflict between the two, ask each what it is that they want to do, document in your engagement letter, and do the returns per their wishes if they agree. If they cannot agree, decline the engagement.
I want to thank you for bringing this up. It was a very good discussion and it made me think about how I would go about it.
Tom
Longview, TX