Another option is to use HumminBirdPC software and sync your waypoints on your PC. This requires you to save waypoints from each unit to an SD card and have a SD card reader for the PC. Fire up the HumminBirdPC software, import the files from the sd cards, then create a file for the set of waypoints and copy all waypoints from both imported files to that new file, sort the waypoints by the coordinates and carefully delete duplicates. Make sure to check all the coordinate numbers for the waypoints so you don’t delete any that are not really duplicates. Then load that file back to the SD cards and put the card in the unit it will prompt you if you want to have them read in. It really is easier than it sounds. HumminBird PC is available from the HumminBird support page where you download the updates for your Bird from.
All this being said my units are networked. Since I fish the same lake most of the time I just do the combining of files once or twice a year.
Winter project – In HumminBirdPC I created files for all the lakes I had old waypoints from and organized them that way. That was a lot more work but HumminBirdPC has the feature to show where a waypoint is using Google Maps. that is a pretty cool feature and really helped do that.