I don't know whether this is OS specific or not, though I know Windows can handle it, since there are programs (e.g. NES emulators) which don't have problem with that exact combo (I configured them to use those keys and they work fine).

I was first using SDL_GetKeyState() but after experiencing problems I started searching on the web about solutions to it.
Most of them recommend to trace events and keep the key state in an array (which I believe that's exactly what SDL_GetKeyState() is doing)

Anyway, I wrote simple routine which displays the events that are received and it doesn't seem right to me, here's the output:

<here I'm pressing and holding 'z'>

> Got 1 events.
> z down

<while holding 'z' I'm pressing right arrow (You know, B button and arrow in Mario to run)>

> Got 1 events.
> right down

<now I want to jump while running, so I press A ('x' button while holding others>

> Got 1 events.
> x down

<up until now everything looks fine, I'm releasing 'x'>

> Got 1 events.
> x up

<and here's the problem, I'm getting those two events right after the previous one despite still holding 'z' and right arrow>

> Got 2 events.
> z up
> right up

What's the reason for it? Am I doing something wrong? Is there any workaround?
Also I disabled all key repeating (though I believe it's like that by default).