Sure you don't have even the smallest thread of wire touching another thread? Since the current conducts on even the smallest fibre of wire, this will send a current to both buttons.
I originally had this problem until I discovered a (very) tiny piece of the wire was touching another one, (the fibre end bits.)
If it is this case, it might be an idea to re-crimp your wires shorter, so that none of is exposed.
(This is just a wild guess on my part, you can see for yourself)