Depends on the Setup.
If you use no DC offset, in that case it does not matter.
If you use a DC offset (best to measure the polarity before the usage with a DC Voltmeter):
Positive Polarity Right hand, negative Polarity left Hand (like it is been said about the standard usage for the Wands of Horus).
But I've done it also in the opposite way because I have an Injury at the right hand and the negative polarity starts the Healing (see Robert O. Becker, Electric Body).
This is also possible.
For more details, please check the link from: