I have no idea if this rumor has any substance to it, but it makes me wonder. I have long wanted a way to view, for example, a document alongside a stopwatch, which is a very simplistic use case, I know. Or a Bible app alongside an ePub reader.
If they do this, I can see an easy way to introduce it with a new OS that will immediately work on a) Retina devices and with b) universal apps.
First of all let me restrict this scenario to a 2/3 style layout display, i.e. One primary app and one smaller secondary app.
If you have your iPad in landscape orientation, you have 2048 pixels across. Even if you halved that resolution you could easily display a second app – as the iPhone mode, in portrait… Even with full Retina resolution. You would have 384 horizontal pixels to spare – at least partly for interface and decoration.
I think the big technical issue there would be scaling down a Retina app on a Retina screen to non-Retina resolutions. It could be done, mathematically, but it won’t be nice, as in, Apple-nice. It has precedent though, pre-Retina iPad apps and games looked just OK, and inflated iPhone apps were workable, and it all lit a fire under developers to provide nicer assets for their apps.
The advantage of using this scheme is that most apps will just work. Those devs with two non-Universal apps will have to develop them or the users might have to have both versions installed on the same device. The interface design adaptations already made for a smaller screen space will pay off immediately when used as a secondary app.
This doesn’t answer all the questions though. What about the vertical blank space on the iPhone-app side? It would seem clumsy to leave exactly 400 vertical pixels unused, not to mention the downscaling we would have already seen on the iPad-app side. Some if the groundwork has been done already, though. Apple has been pushing resolution independence for a long time now, and with the exception if games I think most apps are at least partially prepared for it – and games won’t be played split-screen (let the exceptions fly). If they said “your horizontal resolution is no longer a binary choice between 1536 or 2048” or “your “iPhone” apps will have to allow for a 1536 pixel high screen” then developers could adapt without too much concern. Responsive Web developers have had to deal with this for a long time. Universal app developers have had to deal with this for a long time too. It wouldn’t take long.