From what you have said, there are two sites, each with 5 PCs, and one server. In each case the server runs a WCF service, that some application on the 5 PCs will access.
You said you have a WCF service. You didn't say how you were accessing it - in other words what the client app is.
Supposing that you use WCF client libraries,
You can specify the binding address in configuration (app.exe.config), or in application code.
if you connect via a browser and AJAX, it's just a different URL that you specify.
if you use something else, then... ?
Also I don't understand what you mean by "bypass the firewall". This doesn't bypass any firewalls and I don't think you want to. It might traverse a firewall, but not bypass it.
And, this leaves it up to YOU to figure out how to enable or disable access to the of-site service, whether that is an administrative/privileged user thing, or just a user-selectable address. An example of the latter would be to allow the user to select "Site A" or "Site B" in a dropdown box, within a WPF UI.