Is it possible to get a high efficiency buck-boost converter by switching between the two?
Consider the most efficient type of buck controller; a synchronous type: -



Then, in order to turn it off when the parallel boost section becomes active you need a MOSFET on the output like this: -



Then consider that you need a couple of MOSFETs (or a MOSFET and diode) for the boost section with it's own inductor and it should become clear that there is a simpler solution namely the buck-boost bridge controller.

Take the drawing above and add another MOSFET (shown in red): -



Now compare that drawing with this: -



And this one to understand how it works: -



Explanation here and below is a real chip that does this: -



So, in conclusion, if you begin the process of designing a parallel buck-boost, then if you are shrewd enough you'd notice that the best option is make a H bridge controller because it saves on a MOSFET (or diode) and an inductor and is at least as efficient as what you propose.