Blender will have an option to use mutli GPUs if you have more than one card installed. It does NOT need to be in SLI, but needs to be the same GPU/Family Type. For instance, I have the option to render with both my CPU and GPU since it is a AMD/ATI pair. The same with the benchmark above. Two completely different cards, but Blender can use them both even when they are not paired with Crossfire/SLI, but because they are from the same family of GPU/CPU. The same can be done for nVidia/nVidia, and I'm guessing Intel/nVidia (Can an Intel owner confirm that for me?).
Yes, Blender will see the dual GPU cards as a single card, but will have the option to use 2 threads instead of 1 (if you ever want to use half of the card for some reason).
As for being your gaming system, as long as your best card is your display device, it's not going to effect your games. For the longest time, I ran an ATI card as a display, and nVidia card for rendering. All games would use the ATI card, but Blender specifically used it for rendering. You don't have to have anything plugged into it (display).
Here are some Blender benchmarks for those cards:
GPU: 2x NVidia GTX 980 reference design Gigabyte
2x GPU: 480 x 270 = 00:39.34
1x GPU: 480 x 270 = 01:08.02
BMW v4 (2 BMWs) - Tile size: 480x540
52.55 seconds (GPU at +194 mhz Core/+400 Memory)
Do keep in mind the Titan is being overclocked. I saw another mention it took 1:50 on their Titan Black 6GB, but didn't mention a tile size or anything else.
You can find a lot of benchmark results in the Cycles Testing Thread over at BA.