How can we use Solution Package (WSP) MOSS 2007 to synchronize lists from one server to another?
Copying data / schema from one server to another is not supported and requires custom code.
Is it really necessary that the items 'exist' on both servers? It sounds error prone to me. Maybe it's possible to simply 'aggregate' the items on one server by using a webservice or an RSS feed.
If copying is required then I would create a SharePoint job that runs every x minutes/hours to do the syncronization. Let the custom job communicate with the web services on the other server.
Note : Since your job only runs every x minutes it means that your syncronization is not realtime!
Be carefull with large workloads. Make sure you don't stress your server by trying to synchronize 10.000 every minute.
Have a look at this tool: Content Migration Wizard
It allows you to copy lists from farm to farm using the Migration API. You can also script it to run automatically.