Apple's A5 and Nvidia's Tegra 2 have some fundamental similarities but what else they put into the design that makes all the difference. A chef might add more pepper while I will go more salt. Apple seems to focus on graphics and power management while Nvidia focus on something else, like its home-grown graphics technology.
At the end of the day, the A5, powering the current iPad 2, and the Tegra 2, the brains behind many Android tablets and phones, are both dual-chips. But aside from the OS's themselves that might take advantage of the dual-core nature of the chips, there are not many apps that take advantage of them yet. I reckon only a handful of iOS apps, probably games, take advantage of the A5 and none so far on the Android side.
They are both just too new. So I wonder if some device makers would be prudent to go ahead and continue to design their gears around more efficient tried-and-true single core chips that may held the same user experience.
There could be advantages along this path. So far, I think everything about dual-core could be just hype. I see the advantages of dual-core in Apple's own iOS apps like iMovie and Garageband but outside of that, what? Games?
See, for me, it's about efficiency and longer battery life. And part of having a great mobile experience is being able to use your device how you want and when you want it without worry about the battery running out on you. Designers can build systems that can throttle better based on the needs of the apps or users.
And they can probably design single-core chips with higher clock-speeds that might even rival dual-core chips. With the dearth of apps taking advantage of 2-core designs, an app running on a higher clocked single core chip allows faster completions of tasks versus the same app running on dual core chip but using only one core.
More efficient single core chip running at a higher clock speed means less power used.