On PC's, you can use virtualization to run two operating systems at the same time. This gives you the ability to test a new operating system without actually installing it on your PC's and then having to deal with getting rid of it, or having to replace your current OS. Now, this same feature is making its way to Android, in an experimental project for Galaxy S2.
The people behind the virtualization software Parallels are now trying to make two instances of Android run on the same Galaxy S2 device, through virtualization. This will offer a bunch of benefits to users, although probably mostly to power users, who like to tinker with different operating systems, and try out the new features of those operating systems.
For example, if you could do this on your phone, you could keep the stock ROM on your device, while testing out a new ROM for your device, without losing all your data and settings by replacing the stock ROM with the custom ROM, like people are doing it today.
The other virtualized OS will be using part of the RAM of the device (2GB+ RAM will start becoming handy for that, soon) , but that shared RAM will be isolated, and can only be used by the virtualized OS. Just like on VM's on PC's, you should be able to allocate the number of cores you want for the virtualized OS, too. So quad core chips should be even more useful in the future.
Starting with Cortex A15 we're also getting hardware virtualization, which basically means that virtualized OS will run a lot faster. So if you really want to be able to test different operating systems on Android devices, whether another custom ROM, or Ubuntu Touch or even other operating systems (as long as they are supported by the VM), you should get a device that supports hardware virtualization. Right now, the only chip that supports it is the Cortex A15, but Cortex A53 and Cortex A57 will support it, too, next year, and next-gen chips from Nvidia and Qualcomm should support it, too.
Here's the video with the 2 instances of Android running on the same Galaxy S2, and an explanation below from the project's main developer:
I can assure you that this video states total truth. Indeed, Parallels has strong connection to the project evolvement. The technology's been researched by group of students in Parallels Lab (it is our own educational laboratories in leading Russian Universities) at St. Petersburg University of Russian Academy of Sciences. To be specific, it is an experimental student project supervised by Parallels pros. The technology allows running multiple Android isolated environments on single Android device - effective and scalable with low overhead on virtualization. Yet it is still a technology with plans for further product development.