To combine 2 Master Keyword lists, Export the keywords from each computer and then combine the two files in a text editor app. Now you can Import the new Master Keywords list into each computer.
To save the Master Keywords list as a text file, go to the Metadata Template, or IPTC Info dialog and click the popup menu for the Keywords field. Click the Edit Keywords option at the top to open the Edit Keywords dialog. Click the Export button at the bottom to save the current Master Keywords list as a text file. Use the Import button at the bottom of this dialog to Import your new combined Master Keywords list.
--Bob