nkondrashov The second sensor would be great to facilitate guiding at the same focal length as main sensor.
Bear in mind that you loose the ability to use a reasonable guide exposure time when you use a narrow band filter -- that has always been the problem with the SBIG ST-7.
The idea is patented by SBIG, but the patent has probably expired (the ST-7 has been around for ages; since 1994.) You can find details here:
http://www.company7.com/sbig/products/st7.html
SBIG's patent application was granted in 1996:
https://patentimages.storage.googleapis.com/2b/3d/1f/41b03c5384d960/US5525793.pdf
This is why OAGs are positioned ahead of the filter wheel.
A secondary problem is that the two sensors will need to be co-planar to within a dozen or so microns for both to be in focus simultaneously. The small sensor chips and larger sensor chips don't usually have the same thickness. SBIG has the precision manufacturing and QA required to do it, but ZWO may not.
(This has been tried last year at ZWO, but nothing came out of it, as far as I am aware.)
Chen