@Cyberus
Oh right, so how do you determine which application uses which monitor?
Basically, they act as they both were the same, wide monitor, so you just drag the window over the edge of one of the monitors towards the other monitor and as the window moves over the edge it appears on the other monitor (so parts of a window can be on one monitor and the rest of it on the other). You can set in display properties how the two monitors are physically placed in relation to each other (above, left of, right of, below etc.).