10mcg twice a day is definately not to much as there have been a few years worth of research studies on that particular level with the Byetta folks. I personally take that dose and have no trouble whatsoever.
HOWEVER, you do have to follow the instructions. Once in the morning and once in the evening. I once took two of them way to close together and it made me really really nauseous for about 3-5 hours! That was the extent of the overdose effect but it wasn't pleasant, so I certainly wouldn't advocate taking it in any way other than the way it is prescribed!
But to answer your question 10mcgs twice a day at opposite ends of the day is perfectly safe and effective.