That would be one of those trial and error type situations where you would want to be cautious. Depends on how aggressive the current eel is. If you do try it just keep a close eye on the activities for any serious fighting. Then you will have to seperate them for sure.
One thing I have found that helps with this is to get a rubbermaid (Clear plastic) storage bin with a locking lid and drill 1/2" holes all over the top and sides then just put the new eel in there and let it float around for a few days so they can get used to each other being in the same tank but can't hurt each other.
The rubbermaid idea sounds like a great and safe way to at least give it a shot.
The eel I have now is very aggressive...at least towards me. It darts out of the rocks straight at me when I walk in the room. When the lids are opened his head actually comes out of the water. It's almost scary how bold it is. Definetly keeps me on my toes......
I named him Adolf. When i brought it home at about 8 inches it was in a styro box. I took the lid off and it was in the corner coiled up, after it went after my hand and the net I decided on the name. It has definetly lived up to it. Meanest fish I own by far.