The thing is, I don't get the appeal of side-by-side apps on a tablet. It's one of those ideas that seem useful in theory, but when you have that feature, you rarely use it. In all the years I've had iPads, I can count only a few instances where I've needed two iOS apps open side-by-side. It was usually sufficient to jump back and forth between two apps. And when I really needed two apps open at the same time, I've used two iPads, or my iPad and my iPhone.
And while the OS isn't unimportant, the real battleground is in apps and services. For instance, Apple is way ahead of Microsoft in having an office suite that is optimized for multi-touch input. Telling people "you can run all your legacy desktop apps on this tablet" isn't the way to sell people TABLETS, people will think, "why not just do that on a desktop?" You need to tell people, "here are all these apps made specifically for the tablet" -- and Microsoft is way behind on that front.
Sure, Windows 8 hardware might be getting more powerful, portable, and affordable, but what is really needed for the platform to take off is applications. In the meanwhile, in Jony Ive's secret lab in Cupertino, there probably are all these prototypes of hybrid machines that can switch between iOS and OS X, or run a hybrid OS that combines features of iOS and OS X. Which will happen first, that the Windows camp will put together a compelling set of tablet apps, or Apple will release its hybrid hardware? Personally, I think the rumors of iPad Pro is Apple gearing up to release its hybrid machine. So yes, the next year or two is going to be interesting.
That's an interesting point of view.
In fact, I don't think a tablet would benefit much from two apps opened at the same time. Sometimes it may, but rarely, I would guess.
The real benefit from using side-by-side apps would come when using it as a laptop. In my mind, the line that separates a laptop and a tablet is somewhat blurry. I will explain why.
Four years ago, nobody cared about tablets. Sometimes they were just seen as touch-screen laptops that formed a niche in the market. Then, when the iPad came, the sucess was astonishing, and a new category of products seemed to have been created. But it wasn't. An iPad, and any other tablet, performs a very similar role as a laptop: it is a mobile computing device. You may say a laptop is better for A and a tablet is better for B, but the fact is that if you have only US$ 500 of cash available, you have to choose between them, and one will have to perform the tasks that the other does. And it can be done. It would like be choosing between coffee or tea. They may be different, but, in a way, they perform similar tasks.
And, when these manufacturers come with convertible designs, then I think: well, one can have a device that is both a laptop and a tablet at the same time.
But let's go to the world of real usage. I do have a MacBook Pro and an iPad. The iPad is definitely more portable, but it has its limitations. It doesn't run Microsoft Office, nor any other full-featured office suite which is fully compatible with Office. It doesn't run side-by-side apps. It doesn't even multi-task properly. Then, every day I have to choose one of them to go to work. The iPad is thinner, lighter and more portable. If I spend the whole day in front of my desk, with my desktop within reach, then I could go with the iPad. But then there is a probability that I have an external meeting, and that I need to use Microsoft Word or PowerPoint, or that I have to use side-by-side apps. It can't be done on the iPad. You may of course say that the iPad has Pages or Keynote and that these apps could do the task. They could, but I cannot assure to my clients 100% compatibility with Microsoft Office, and every client expects to receive files which are 100% compatible with Microsoft Office. In the end, I always take the MacBook Pro with me, and the iPad stays home. Then, if I have to travel, is even worse. What if I have to do some serious work stuff, and I only have the iPad with me? That's why I always take the MacBook Pro. And the iPad goes in the backseat all the time. In the end, I feel that I hardly use my iPad. I don't even know where it is right now.
If you forget for a minute the categorization of these products as laptops or tablets, and think of them as devices, then the iPad will be a very well-designed device, but very limited in what it can do.
Other people may not have the same needs as me, but perhaps they will soon perceive that they don't actually need both a laptop and a tablet. Just one device could do all they need. But if the iPad is too limited, people may start shifting to Windows devices instead. Some of them are already very compelling. As you said, Apple may well be working on something to unite its tablets and laptops. The idea is certainly interesting and, if it has not yet been correctyl done, it's probably because the current state of tech does not allow it. But it will soon.