You can only use one racquet PER POINT, but you can switch between points as often as you like. (So, for instance, if a string breaks mid-point you cannot switch racquets and continue with that point. But you can switch racquets once the point is over. The exception would be if you break a string on a first serve that's out - you can switch racquets for the 2nd serve.)
Anyway, I change racquets every change-over. My 2 racquets are identical - the change just lets me get one with a dryer grip.