It's nothing new, the shoebox shockers were stacked tube spools. The only difference is how they move the bolt. The shockers just put the bolt actuation right behind the bolt rather than below. They looked significantly different because the packaging was not nearly as optimized...but the basics were there. Bolt up top, spool below/side (my PVI was to the side).
The packaging has been shuffled a little...but the idea of using a spool dump chamber and independent bolt actuation has been around for a while. Efficiency should be on par with any other spool gun (ie not as good).
I think the old Nova's worked this way (distinct bolt & dump chamber controls), maybe there are more yet that I'm missing.