If you add a second pump for the second GTX, you could also add a second rad like the MCR220 and just put it on a second loop. Doing that the second pump could be much smaller - like an mcp355. Wouldn't need a res you can just do a T, put two low speed yate loons on the second rad and you'd have a fully functional easy to maintain second loop that should cool a gcard just fine.
LOL you do realize this is an Antec Sonata II case. I'm pushing it with mounting a single loop, and the 2nd pump would have to be in the same loop I could not imagine running a second loop in the computer. Honestly I think I would go after an extreme 24v pump like the Iwaki 20ZVT or whatever it is labeled and sell off the one I have if it didn't cool well enough. For right now I'm going single GTX, because the game I'm playing I need to run dual screens.