You can get excellent results either way. You can tap with your right hand, your left hand, switch sides (right collarbone spot, under left arm), or tap on both sides simultaneously with both hands. I let my intuition and my situation guide how I tap. If a little more “juice” is needed, I’ll tap both sides at the same time using both hands. If I am in a more meditative state, or driving the car, I’ll use one hand.
Tapping with both hands does have the advantage that you are almost certain to tap the proper spot on at least one side. For the same reason, we tap with more than one finger. When teaching people new to EFT, I demonstrate with both hands.
In recent (Sep 2004) demonstrations, Gary Craig often (but not always) used both hands when tapping with participants who had chronic disease. Also, EFT practitioners with a background in Chinese medicine highly recommend using both hands, as does Dr. Mercola.
The good news is that EFT is very forgiving. If you are not getting the benefits you expect, you can always try tapping with both hands, tapping more times at each spot, using a different tapping rhythm (slower or faster) and pressure (lighter or stronger), and using more emphasis with your statements. What I find is that being specific and using emphasis are the keys to getting consistent results.