Easiest ways would be to Export the Nav Data from the unit containing the waypoints you want and then open this file in HumminbirdPC.
Once there you can make a copy of the Nav Data (will be shown as a GPX file) and delete the unwanted waypoints, routes and tracks, than upload it to a another memory card to be installed in the other unit.
As an option to this: once you have the Nav Data in HumminbirdPC as a GPS file you could open a New File and simply drag and drop copies of the required waypoints into the new file. The new file gets uploaded to another memory card and installed in the other unit.