The first stage is to get both SPI displays to work in clone mode. I will call this first version "Mark I" :)
First, we have to physically connect them.
Then, we have to build a new dual_hy28b_nots overlay. Display and touchpanel both use a SPI chip select (CS) line and Raspberry Pi have only one SPI channel with two CS lines, so we have to remove touchscreens in order to activate both displays.
Finally, we will add some modifications to X11 to allow clone view with both SPI displays.
Full guide is available in french on this link
I think you could easily follow the quotes, if not, please send me a message !