Understanding disk requirement to make fail over cluster
That all depends on which configuration you're seeking. As @EricHiggins noted, a traditional cluster will have shared resources, including disk, network name(s) and IP(s), and the service itself. SQL Server is a cluster-aware application and will interact with the Windows cluster service. When a failure (or manual failover) is detected by the cluster service, the SQL Server service is stopped (taken offline) and the instance resources (name, IP, disk, and other cluster resources) are migrated/transferred to the another node in the cluster. When the resources are fully transferred, the SQL Server service is started and the instance is available.

With SQL Server 2012 Enterprise Edition, the concept of Availability Groups was introduced to the system. Availability Groups are a shared-nothing environment that uses failover clustering for network naming and listening, but uses mirroring to protect at the database level. 

There are many more details to go into to draw out the distinction between (and the benefits of) each of these solutions. However, to directly answer your question, if you're going to cluster using failover clustering then the TempDB LUN would be shared. If you're going with Availability Groups, then you'll double your storage, but be able to protect at the database level (instead of the instance level). 

I'd need more information about "appears as one" statement - to what, or whom, does it "appear as one?"