I call xQueueSendFromISR twice posting to two different queues. But only one task waiting on the queue gets awaken.

    
__attribute__((__naked__)) static void NAME(void) { 
    portENTER_SWITCHING_ISR(); \
    portENTER_CRITICAL();
    bool taskAwaken = FALSE;
    taskAwaken = xQueueSendFromISR(_queue_1, &key, taskAwaken);
    taskAwaken = xQueueSendFromISR(_queue_2, &key, taskAwaken);
    portEXIT_CRITICAL();
    return taskAwaken;
    portEXIT_SWITCHING_ISR(); 
} 

if I only post to one of two queues - the corresponding task is awaken as expected.

What am I doing wrong?

I use preemptive scheduling. Port: Atmel UC3A3256

While I am not familiar with that port, the signature of xQueueSendFromISR changed in Version 5 to
portBASE_TYPE xQueueSendFromISR(
xQueueHandle pxQueue,
const void *pvItemToQueue,
portBASE_TYPE *pxHigherPriorityTaskWoken
);
where the return value is NOT the new value of taskAwaken but a flag to indicate if the Send was successful or not., thus your lines should be (not also the adding of an & when passing taskAwaken)

flag = xQueueSendFromISR(_queue_1, &key, &taskAwaken);
flag = xQueueSendFromISR(_queue_2, &key, &taskAwaken);