How best to organize a few related sites

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Hello,

I have a couple forums that are related and ideally are able to share some resources: users table db, some css and image assets, templates, etc

The way I see it there two options:

  1. Everything on it’s own domain

Definitely the easiest way but there may be some resources duplicated. Is it possible to share a database with the other sites?

  1. Create an “umbrella” domain and the actual sites will be sub-domains of the umbrella site

I would be able to keep all the sites together. Easier sharing of resources. But would I be creating dependencies if there is too much sharing?

How do the big corps(Google, Yahoo) handle organizing so many sites?

Any help is really appreciated!
Thank you