It depends on the second species of eel. If they are of comparable size, some eels will sit well with each other. Although I am of the opinion that it is only a matter of time before they will fight. I have kept eels together before, but inevitably they will eventually fight amongst each other and one will show up injured. I'd say not to do it. But if you're going to, check to make sure they are the same size, and make sure the two eel species are fairly passive to other eels/fish.