it depends on the game

for instance if its something like comand and conquer then you can't scroll in the direction of your second monitor (so if number 2 monitor is on your right you can't scroll on your right), so you can still do things like instant messaging

if its something like warcraft (the rts, not the mmo) then you have to alt+tab or do something similar that will have the game minimized

otherwise the only way you can still do other stuff without quitting or minimizing the game is to play it in a window

some games support this i think (like dark reign 2)