There isn't a "right" answer. You can ask them for two different letters -- they upload them into two different slots. Most letter writers won't be insulted. Or, you can ask them to write a generic letter and not mention the field. If you do get two letters, you need to make sure that the right letters get assigned to the right programs. And, you have to hope that the letter writers (or their secretaries) upload the correct letters to the correct slots. Every year on SDN there's someone's sob story about how a sec uploaded the letters backwards, and then they sent the wrong letters to every program.