Diagonal C-stick button?

Hi fellow modders, i've been tinkering with the gamcubecontroller for a while now and i just can't seem to find out how to make a button input a diagonal c-stick input. I got the pin for downwards and left (or right not entirely sure here..). Grounding both at the same time won't result in my desired input. What i want in long term, is to get my X Button making that input.

Couldn't find anything on the webs, anybody know how to make it possible?

My pcb looks exactly like this one here

http://s38.photobucket.com/user/Lexan2002/media/Old gamecube controller/IMG_20141121_233256.jpg.html