Some tasks are sequential by nature. To make a cake you need to crack some eggs, add some flour, make the cake mix etc, and then at the end put it into the oven. You can’t put the cake tin into the oven until the cake mix is ready. So even if you had two chefs in a kitchen you can’t necessarily save time on one task. There are steps to be followed and the order can’t be broken. You can multi-task, in that while one chef is making the cake the other can prepare a salad, but tasks which have a predefined sequence can’t benefit from dual-core processors or even 12 core processors.