I regularly have two documents open on my screen and I try to make notes in one while I read the other. Having a big widescreen monitor helps. Twin monitors are even better but there is an annoying windows feature that slows things up. Only one window can be active at a time (has focus) and if you want to do anything to the other window you must activate it by clicking it. Typically this means clicking a window to scroll up the text then clicking the other window to start typing again. It doesn't sound like much but the constant switching of focus disrupts my work flow and inevitably I get it wrong occasionally and try to type in the wrong place.
WizMouse from Antibody software is a simple, fix for this with an unlimited free trial. When it is active then the mouse wheel controls the window under the pointer WITHOUT CHANGING FOCUS. In practise this allows you to keep typing away in the active window while using the mouse wheel to scroll through the other window. Brilliant.