I used both electrodes on the same wrist with great success.
I accomplished this using TENS pads(cut to size) combined with a high quality conductive jelly(to avoid irritation) without any issues whatsoever. Though I'd also add that when I ended-up with burns when I used the silver plates covered in cotton covers. Which was solved by using TENS pads(silver/carbon) with conductive jelly. And the reason for this is where the combination of the two improves conductivity and the heat generated by the resistance standing between the human skin and the electrodes.
Hope this helps.
PS, the circuit I use runs at 30v rather than the usual 27v standard(improved beck circuit). Which wasn't necessary for anything other than when I used it on my ankles at night. Otherwise, I can't tolerate the signal beyond 60% using the pads and jelly.