You would use one accounting tool for the FEIN's financial data. You would use whatever they use, when you are working with them cooperatively, so that there is a provision for data to go back and forth. You would use whatever tool you want, when you are working with their data and your office is solely responsible for managing their data on behalf of the client, and typically presenting via reporting. You would not mix accounting packages for the same data; there is no need to use more than one tool of the type.

You would not use POS, but learn to understand their use of their POS and their data reporting. I bring this up because I support a few QB consultants (Pro Advisors) with clients using POS, and more often than not, the problem we are researching comes down to the end users not understanding the requirements for POS operation, not using the checkout process correctly, not handling the sales taxes setup, and then the accounting is a mess.

For instance, a "The UPS Store" franchise has the requirement to use the UPS software for POS and the accounting structure integrates to the QB file, but that has very specific requirements you cannot mess with. Print shops and NAPA, etc, have similar requirements.