If you have 2 servers, then you have 2 different XMPP domain names. How do you plan on managing the roster if you get it so users can log into either server? It seems overly complicated and error prone for users. You might consider just one server, and disallow all file transfers, if that is your major concern. Otherwise, it sounds like managing your roster will be a complete mess unless you only have 10 people.
However from what i have read, you can tell the server not accept file transfers, but wont the spark cleint then just transfer files between themselves, or do they not do this if they are on different switched networks ?
Something to keep in mind is that by allowing information exchange at all, your no file transfer policy is purely cosmetic. As are most of the methods to stop file transfers in spark/openfire. They work provided the clients stay within the box defined for them (and many will, especially with proper user education). But determined individuals will still find a way to make it happen.