Why can I measure two pins simultaneously?
You can test it easily by calling the function twice directly after each other. If the first time a true is returned and the second time a false, the state is internally reset.

But probably because after you call the fell function, the internal state is reset, so after the call not a new fell has been happened.

So store the return value of the fell and rose functions and use those instead.

Update:

I'm not sure if this is the actual implementation but I found for fell/rose and see this:

bool Bounce::rose()  
{  
    return ( state &amp; _BV(DEBOUNCED_STATE) ) &amp;&amp; ( state &amp; _BV(STATE_CHANGED));  
}  

bool Bounce::fell()  
{  
    return !( state &amp; _BV(DEBOUNCED_STATE) ) &amp;&amp; ( state &amp; _BV(STATE_CHANGED));  
}  


So as long as the state has not changed it returns false (which is similar/equal as 'resetting the internal state'. Except this reset is done at a different location (but checked in the fell and rose function).

Update:

See remarks from gre_gor below