If you are comfortable with using Terminal to execute a few commands, you can set up BaseCamp to share its data between multiple computers. Note that this is a Mac-only solution. Download and install the free Dropbox application from www.dropbox.com. If BaseCamp is currently running, quit it. In Finder, go to <your Home directory>/Library/Application Support/Garmin. Move the BaseCamp folder from that directory to your Dropbox directory. Launch Terminal. Enter the following two commands:
cd <your Home directory>/Library/Application\ Support/Garmin
ln -s <your Dropbox directory>/BaseCamp
Now go to another computer that you want to sync BaseCamp data to. Install Dropbox. Again, quit BaseCamp if it's running. In Finder, go to <your Home directory>/Library/Application Support/Garmin. Delete the BaseCamp folder. Note that any BaseCamp data that was on this computer that was not on the other computer will be lost here. So, you may want to manually sync that data before you do all this. Next, launch Terminal and repeat the two commands shown above. That's it. Your BaseCamp data will now be automatically synchronized between the two computers. If you have a third (or fourth, etc.) computer that you want to sync to just repeat the second part of these instructions on that computer.
BIG WARNING!!!!
Make sure that you run BaseCamp on only one computer at a time. If you run BaseCamp simultaneously on multiple computers that are syncing data this way, it is very likely that your data will be corrupted or completely lost. The easy way to protect against this is to quit BaseCamp on each synced computer when you are done using it on that computer.