It's hard to do because it isn't really that useful. Mac OS is a symmetrical multi-processing system. The operating system task scheduler is free to put any task that needs the CPU on any CPU at any time. The only real penalty to moving a task to a different CPU is possible CPU cache inefficiency. But it is outweighed by the fact a CPU is available to service the waiting task.
It's not like Safari only runs on CPU 1. As a matter of fact many of these programs are multi-threaded and it's possible Safari could have multiple threads running on multiple CPUs at once.