I have been looking at that system, but I don't have experience with it yet. You definitely can purchase a second VGA Water Block (ZM-GWB2) separately.

My inexperienced opinion (for what it's worth):
I don't see why you couldn't add the second VGA block to the cooling loop. It is only another water block and a short length of tubing, and I don't think the extra volume of coolant it needs will be a big deal for the reservoir or pump. The block is less than an inch tall, too. I do wonder if the tube orientation will want to occupy the same space as the SLI bridge. Hmmm...I'd ask Zalman what they think.