Electronic drum set powered with batteries
Is it safe to use couple of parallel joined 9v 500 mA batteries ...


Batteries are rated in mAh - not mA. If they are 900 mAh then they can (ideally) provide 900 mA for one hour or, more likely, 90 mA for 10 hours with somewhat less at higher discharge rates.

To estimate run time you need to measure the current drawn by your drum set. To do this you need to somehow wire a multimeter on DC amps range in series with the power supply. You can estimate your battery run time by \$ h = \frac {battery\;rating}{meter\;reading} \$ where h is hours.

You can parallel the batteries if they are the same type.