Make sure all maps have an API first! Then what you need are event listeners, especially for the dragstart, drag and dragend events. (These are from Google Maps API v3)
If you declare an event listener for an event, say for the drag event, there is an object returned in the function. Again, in google maps api v3 it is the event object, which has the latLng property which is an object of google.maps.LatLng type (ergo this contains the coordinate).
So you should declare these event listeners for all the map instances on your page and you should include commands to change all the other map instance's center/zoom etc. in each of the event listener functions.
I hope that the above made sense to you but if you have any questions feel free to ping me!