Drilling your second block isn't a good idea. Voiding your warranty and leaks comes to mind.
Connecting the video cards in series like you diagrammed will leave one of them hotter than the other.
Running them in parallel like I diagrammed and your blocks seem to be setup for will raise the temperature of each card a bit but they will both be the same temp and you'll have less back-pressure from the two blocks.
OH yeah!! Your video cards monitor their fan speed. Before you take off and replace their heatsinks you'll need to disable the monitoring feature in their BIOS through the video driver, or your machine will not boot!