An already fixed very non dominent one of eitheir sex. I'd do introductions first to make sure it will work too. Its always really iffy throwing in a same sex. Personally I think females fight worse than males but thats just with my dogs. you may have to keep two apart at all times so just know that before you get another.
My cockers both came from sheletrs. Thats where I will be getting my next one from.My male is ( years old and Im not sure how he would do with another young male.My female is 3 and she is very dominit. If I got a female would I get a submissive? Thakn you soo much for the advice!