Question about comp-adder vs conditional sum adder

something i didnt understand about comp-adder cost , you said that comp adder has lower cost in compare to conditional sum adder but the wat i see it -comp adder “works” like two conditional sum adder and therefore when you said that you get less cost you actually get more cost cause 2 comp-adder works like 4 conditional sum adder (the way i see it) so instead of function of 3 recurusion calls (onditional sum adder) ,you have comp-adder that “include” 4 recurusion calls, probably i didnt really understood the comp-adder functionality , can you explain to me? thanks!