Re: two zappers at once

Even with a full blast of harmonics going from one zapper to the next, there is nothing to fear.
We talk about 3 to 4mA peak with a zapper (main signal + harmonics). A 555 can deliver up to 150mA and (most of the times) is protected by a 1000_Ohms output resistor. Not much to fear on zapper side, with the zappers you have used.

On your side, it could be somehow different.
With only one zapper, you can have hard and deep side effects due to die off, if your elimination channels are not clean (not an obligation, but ...)
With two zappers at the same time, you are rendering the elimination problem MUCH worse.
Be careful : MORE and/or FASTER is not always better… You have to take in account the capacity of your body to follow the zapper’s action.

Sometimes, two similar devices are used on the same area to hit VERY HARD the cross section of both independent currents. This kind of technique is to be used with great care, and by health professionals only.