Automatic selection in multiselect picklist

I have a Region Form which contains a multi-select picklist of Territories..

I have an another user master where there is two multi-select picklists: "Region Name" and "Territory Name".

Now, while I'm selecting some regions in the user list, the territories under that region should automatically get selected, i.e; in the user form territory should be selected automatically while I'm selecting the region.
How can I do this? It would be very helpful if anyone put some deluge scripts/sample application for multiselect picklist (not single select picklist) regarding this issue.

Thanks and Regards
Sudhakar Saha