Hi, both is managable...
If you have two tasks, second task should be started when first task is completed of 75%, than set the gap between these two activities to -25%

Sure you can divide some task. I am not sure of English name of that button, but its button on standard toolbar, the name should be something like Divide task. Other option how to do that is to go to the Resource usage view and enter zero days or zero hours in place where you want to divide it...