I am not entirely sure how your code is structured, but if you currently have a data or persistence layer, or at least a database access class where all your SQL is executed through, you could override the save functions to write changes to both databases. If you do not have a data layer, you may want to considering writing one before starting the transition.
Otherwise, you could add triggers in MSSQL to update Hadoop, not sure what you can do in Hadoop to keep MSSQL in-sync.
Or, you could have a process that runs every x minutes, that manually syncs the two databases.
Personally, I would try to avoid trying to maintain two databases of record. Moving changes from a new, experimental database to your stable database seems risky. You stand the chance of corrupting your stable system. Instead, I would write a convertor to move data from your relational DB to Hadoop. Then every night or so, copy your data into Hadoop and use it for the development and testing of your new system. I think test users would understand if you said your beta version is just a test playground, and won't effect your live product. If you plan on making major changes to your UI and fear some will not want to transition to 2.0, then you might be trying to tackle too much at once.
Those are the solutions I came up with... Good luck!