Yes single IP.
It's important to note that while you "can" install REST on 2012, there are a few things like Order Confirmations that will not work with .NET 4.5. We've had to rollout a separate API instance with IIS 7 and Windows Server 2008 R2 to only handle order confirmations from Tessitura. Normal SOAP traffic will work fine with IIS 8 and 2012 (we haven't found anything amiss yet).
Also, if you do get errors about binding for REST services such as the Constituent Server/Transaction Service or Reference Data Service I found a fix to put in place would be to edit the web.config for only these three as follows:
In the section labelled <serviceBehaviors>:
old:
<behavior>
<dataContractSerializer maxItemsinObjectGraph=20971520" />
<serviceAuthorization serviceAuthorizationManagerType="Tessitura.Service.Common.Interceptor.ServiceAuthorizationManager, Tessitura.Services.Common" />
</behavior>
new (add a name):
<behavior name="REST">
the reason for the name is both the endpointbehavior and the service behavior have the same binding name. No big deal once you figure it out..just ran into this myself the other day.