Apple has never shied away from copying a competitor's feature, if it's a good one. Indeed, a good one that Windows offers is the ability to run a couple of apps simultaneously in separate windows. There are many times when I am writing a blog post, such as this, on my iPad and I need to browse the web for a photo or text to quote. Switching between apps is cumbersome. Microsoft found a way around it, and they have been using that distinction in advertising. It's a valid differentiator. However, that may change soon, according to 9to5mac.
Apple is expected to introduce a new version of iOS at their Worldwide Developers Conference, in June. WWDC would be the perfect time to announce this split-screen personality, as developers will likely need to rewrite their app to accommodate the smaller resolution. One approach is to switch to iPhone mode and run the two apps side by side on an iPad. That would require little to no effort on the behalf of developers. But it would not be as elegant as it should be. I would expect Apple to craft a specification that developers could use to make their apps splittable.
There have long been rumors of an iPad Pro, which would be a more professional iPad, perhaps with a 12" screen and a keyboard similar to the Windows Surface. This is where the iPad would bump up against Apple's MacBook Air. If that happens, multi-windowing would be even more important. While the split-screen may be a temporary marketing advantage for Microsoft, the real advantage for Apple is being able to see competitors' approaches and then cherry picking the best features and making them their own. As long as they don't violate any patents!