I would not put another one in the same tank with the one you have. With all the research I have done (a lot since I'm thinking about getting one) says that two in the same tank will fight unless it is male and female.
I you want some cories then get 3 of them. They like to be in schools. Rainbow sharks get more aggresive as the get older and get too big for a 10g. I think that if he was in there when small and then moved to a bigger tank it would work out.
I was thinking about getting one for my 29g. I don't know. Still thinking about it.