Chipworks did a first round of analysis on Omnivision split pixel sensor back in April and kindly agreed to shed some light on the subject even though they haven't gone forward with a full analysis. Chipworks has posted a flyer based on the brief analysis, a custom landing page can be found here. (This page is temporary, so keep the pdf file, if you need it.)
The design is quite interesting. Each pixel has two photodiodes per each 6um pixel, as depicted below:
Each split pixel has dual microlens above, one per each photodiode (I think Visera spent many sleepless nights to optimize these oval lens shapes):
It looks to me that Omnivision used dual exposure technique to achieve its goal of 110db DR with this sensor. Two interleaved rolling shutters can run across the array, where reset curtain is followed by two read curtains - short and long one. Then the two readouts are combined into a single HDR image. Obviously, there should a large memory to hold the intermediate frame results. The die photograph in Chipworks document supports this guess, showing a huge memory-dominated digital portion.
So, to me this looks like a dual exposure sensor with on-chip HDR reconstruction. Its main competition are Pixim digital pixel and Toshiba-like dual exposure pixels with charge skimming.
In principle Pixim can get better dynamic range, but I believe Omnivision 4T pixel is better in low light, especially if two photodiodes operate in normal mode, with same exposure on the two photodiodes.
Comparing it with Toshiba solution, I'm less sure about Omnivision advantages. Toshiba sensor might have some linearity artifacts when connecting short and long exposure ranges. Omnivision probably does not have this. However, Toshiba has an advantage in low-light sensitivity in HDR mode because Omnivision sensor effectively loses half of the light for the short exposured photodiode.
One question that bothers me though, is that the two photodiodes are spacially shifted relative each other. So, there might be some color artifacts when interpolating the edges on HDR picture. It's probably not something fundamental, but might take some effort to correct.
Many thanks to R.F. from Chipworks for providing this presentation!