I am working on a web server logger but while Accessing to to the data via web browser the logging is not done, I am think on implement the logger in one UNO ant the web server in other UNO.
While the web server has the file open for read, the logger can't open it for write. So, it really doesn't matter how many Arduinos you throw at the problem. You need to redesign your application so that the web server can serve up the file in between the times that logging occurs.