As i understand, you're using the keyboard, so your problem can be solved by:
a). (temporary) assigning both buttons to the same key
(or maybe try assigning them to another keys
- when I was playing CT on snes9x and had to press 3 buttons
at the same time, and couldn't do above, same keyboard
didn't accepted A,Z,D but accepted Q,W,T IIRC)
b). (permanent) sad to say but - buy a pad