if the other button is x, why would you make the second action button y? they are so far apart on the keyboard, z and x have been used in emulators and flash games this way for years, are you just now learning about the internet?
I think I can explain, where the confusion lies in here and why the choice of X and Z as default buttons is a bad one.
Anglo-American other countries use the QWERTY keyboard layout, where X and Y are conveniently placed alongside each other. Many other countries however use other keyboard layouts. In the German language area, for example, the position of Y and Z is switched when compared to QWERTY (the layout used in those countries is called QWERTZ because of that). So having X and Z as default action buttons will work fine, if you live in the UK, the US, Canada, Australia etc, but it is a horrible choice on QWERTZ keyboards.
Cogito ergo sum: I would politely ask ASMB Games to remap the default action keys. S and D or A and D would be a good choice for example, as those letters are placed on the same position on basically any widespread keyboard layout. Which is also the reason why W, A, S and D are the standard movement keys for every ego-shooter game on the PC ;)