Specifying the default database to store new field tables in
Unfortunately, Drupal was not designed to work this way.  The only way to figure out where to put each table would be to carefully inspect each new module and adjust your settings in advance of enabling the module. Even if you manage to do this, you are likely to run into problems with modules that stores critical information in the variables table.  Clearly you cannot share the entire variables table, so you would instead need to figure out how to synchronize individual values that must be shared, avoid race conditions, etc.

You should strongly consider using domain access or subdomain to share node content between different sites.  You will have some work ahead of you in converting from your current site format to one that will work for you; the migrate module may help with this task.