Combine ADSL with Leased Line

We are a small company of around 25 users. However our users tend to be very heavy users of the internet. With a limited budget, I would like to improve on our existing 256 kbps leased line. My thought was to combine the guaranteed bandwidth we already have with a 2 mbps ADSL connection, thereby achieving a good increase in speed while retaining the reliability of the leased line. Does anyone have any ideas how this can be achieved?