A hyperthreaded cpu actually emulates an extra core. A cpu with a single physical core and a hyperthreaded core is seen by the OS as two cores, it doesn't know anything about the hyperthreading. This means that the load average 2.0 means the system is fully loaded, not 1.0.

Although, it is much easier to go from 1.0 to 2.0, as opposed to 0 to 1.0 on such a cpu, because the cpu can't handle much more work before it gets overloaded.