The Ares II is a dual-GPU card that's much more powerful than a Titan. But the Ares II also much more expensive than a Titan, so you'd have to compare apples to apples, such as a PAIR of Titans.
The Ares II even beats other dual-GPU cards like the Radeon HD 7990 and GeForce GTX 690 in most titles, though not all. For example in Skyrim the GTX 690 still outperforms the Ares II.
http://www.bit-tech.net/hardware/graphics/2013/02/12/asus-ares-ii-review/1
For multi-screen gaming, multiple Titans are better than a single GTX 690 or single Radeon HD 7990 because the SLI scaling is much better. But the Titan is still only a single GPU card, it's not the most powerful card from Nvidia- the GTX 690 is.
Here's the bottom line on the Titan:
http://www.tomshardware.com/reviews/geforce-gtx-titan-gk110-review,3438-7.html
And here's the lowdown on other high-end cards (excluding the Ares II, which is really a better cooled version of the 7970X2)
http://www.tomshardware.com/reviews/radeon-hd-7990-devil13-7970-x2,3329.html