There are at least three different ways to configure the hosting of a WCF Service Application inside a Web Application and Scott Marlowe takes a walk through each of these methods in his post.
If you want to use the WCF service across two or more web sites you generally don't want to have duplicate code existing in two different places. You can create a base class, and derive each service from that, but I'd rather just have the entire service contained within a single project. Then it's just a matter of deploying the service onto the different web sites (or web projects). One way of doing that is to copy the service's contract, implementation, and config into the web project.