using it on the same wrist requires an application of 3 milliamps of current, enough to burn the skin. That is because most of the current just goes straight across to the other electrode instead of entering the arteries.
using it on one wrist requires only .14 milliamps (1/21 as much as 3mA) which is gentle on the skin and does no interference with the heart.
Possibly the best method is ankle to ankle but with most people the artery is hard to find there and the skin is more sensitive than at the wrist.
Most of the blood electrifiers are set up to be used only on one wrist. That was the final Beck method which he changed to after the feds started hassling him and looking for any little reason to put him in prison. (Message me if you want to know which blood electrifier is set up to be used on both wrists or ankles and indicates when the user is receiving .14mA of current)