So disassamble the two (cables not in picture):

Quite differend design, but nevermind. So I want all internal parts from (black) USB mouse into the grey housing, the PCB part with the buttons should stay in place, therefore I used the original ones by cutting of the front part of that PCB and soldered them im parallel to the other buttons:

Yes, perhaps 2 wires each buttons would have done the trick also, but to be sure I wired them all. After that I did a short test and all buttons worked fine.