I share a data directory among three machines. Technically is very simple, you can use the technology that suits your use case. In my case, I use Dropbox with its syncronization service. You can even share some directories or files but not others; In my case I share activities, config dirs, and almost everything but some specific files (the ones related to sensors for training, the rideDB database or something else.
But, be aware that GoldenCheetah is not meant to work in such way, so it is under your own risk; it is not considering that files can be shared multi process or multi machines at all. In my experience, I use it carefully: I never open two instances at the same time, and I wait for the data to be well synchronized among machines to open any instance of GC.
Hope this gives you an idea of the options you have