Depends on the confidence you have in that doc, and the one that the next one is scheduled with.
Since this one is a neuromuscular specialist, I would tend to stick with him and go for the next EMG he wants to do. It is a different type and might show something, or nothing. Most likely he knew what he was doing and you will not get a different result in such a short period of time.
Is the next one with a nm specialist? If so I might consider it just for peace of mind if cost is not a factor. However, I would not expect a different result. My $.02.
Also, if you get face time with the next doc, and you believe in his abilities, he might recognize or go down a path with your symptoms that could help you. This stuff is weird and each doc processes info differently, so a 2nd opinion never hurts.