The correct term is superimposed
load firearms

, just like the title of D.R. Baxer's standard reference work (hard to find and extremely (!) expensive - I paid 300 euro; a copy is in my own library).
P.S. I just noted it's offered at 180 € at abebooks ...
The reason of course is that the loads in these guns were superimposed, not the firearms themselves.
BTW, sometimes the SEARCH button on the forum is of some help

- please see my threads:
http://www.vikingsword.com/vb/showth...erimposed+load
http://www.vikingsword.com/vb/showth...erimposed+load
http://www.vikingsword.com/vb/showth...erimposed+load
Attached please find a scheme of the superimposed load principle of a four-barreled mid-17th c. wheellock carbine, from D.R. Baxter, 1966.
Best,
m