Will using two tens units at the same time in the same area deliver more voltage than using one?

 

It depends on the design of the units.

 

Will using two tens units at the same time in the same area deliver more voltage than using one?

Absolutely not.

Tens units are designed to deliver a certain level of electrical stimulation in the form of current to a given area.
Each tens unit has option settings to select the level and pattern of the electrical stimulation. They are designed to not exceed established levels.

You would be best to consult your health-care practitioner before attempting to experiment on your own.

 

Will using two tens units at the same time in the same area deliver more voltage than using one?

If more voltage was desirable, it would be built in to the TENS unit to begin with. The therapeutic value does not depend so much on the voltage as the frequency pattern, the size and positioning of the pads, and other factors.

In my experience, the cheap TENS units work but resemble toys. The TENS treatment I got from my chiropractor was an order of magnitude more ... invasive. And effective.

What are you looking for, an increased effectiveness?