What would be the output of a 555 multivibrator ic in bistable mode when both the trigger and the reset are set low simultaneously?
From 555 datasheet (TI)


  During the timing cycle when the output is high, the further application of a trigger pulse will not effect the circuit so long as the trigger input is returned high at least 10 μs before the end of the timing interval. However the circuit can be reset during this time by the application of a negative pulse to the reset terminal (pin 4). The output will then remain in the low state until a trigger pulse is again applied.


Ultimately this is not specific if Reset is active and Trigger is applied. 

From 555 datasheet (Fairchild)


  When the low signal input is applied to the reset terminal, the timer output remains low regardless of the threshold voltage or the trigger voltage. Only when the high signal is applied to the reset terminal, the timer's output changes according to threshold voltage and trigger voltage.




Every block diagram shows the 555 logic as a flip-flop.  Reset (active low) will always override trigger.