Belquette tried this out years ago with the Mod 2 . First printer prints the white then auto rewinds to the second to print the colour.
I assume in the end it just makes more sense to have 2 separate printers. I've gone around and around in my head on this and ultimately come back to the effort isn't worth the benefit. Might as well do 2 machines. Now if you custom built from the ground up with 2 printheads that is nice because you get a faster machine in the same size printer but just putting 2 printers inline you don't save much on space.