I don't know about the 2 buttons. Just more moving parts to wear out. I can also see a lot of unintentional app switching. I wish they would do away with the hard home button and go with a soft, capacitive home button.
I doubt they will change the single home button any time soon. It reflects the simplicity of the design and has become iconic for iDevices. I think they will also stick with the same dimensions. Innovation becomes extremely difficult when you lead the pack - you risk annoying your customers with new features added for the primary reason to differentiate the product fron the last one (Microsoft). Personally, I think the next major advances will be within software, connectivity and interoperability not hardware.
Never underestimate the power of stupid people in large groups.