Re: Well, those are threads not physical cores.
Yes -- the OP is incorrect ; execution resources are shared and not particularly increased
for the second thread. The reg files are doubled up, but it is about utilising at higher fraction
of a fixed superscalar resource.
However, most cores on most code are idle most of the time. Waiting on memory. The sharing of execution resources isn't really why most people see "hype threads" as not working so well.
The more fundamental problem is the sharing of the L1 cache and the bus to L2. L1
thrashing in particular can be as painful to watch as two keystone cops vying for the same door
and neither making it through.