Detach the application from the database

I try to figure how to detach the database from the application. I’m a DBA, I work for a software develpment company. We have client that use our software with both SQL Server and Oracle. How can we get ride of developing two applications, one for Oracle, one for SQL Server? There’s surely a way, my idea is to using a DLL for all actions within the database, and just change the DLL to change the database engine (each DLL optimized for the database concerned). Is someone already tried this? Or do you have a better solution? Don’t tell me it’s impossible, nothing is impossible, sometimes it’s just too much work for what it worth …

Thank you