Installed Home Premium on an HP workstation which has 2 physical CPU's. Only recognising a single CPU in task manager but there are 2 listed in device manager?
I do not have multiple processors, but there is an option in Task Manager to show one graph for all CPUs or a graph for each CPU. Check under View, CPU history.
Depending on the technology in the chip it can show the wrong amount of cores too, for instance anything with hyperthreading will show double cores than it actually has.