Are they spayed? Mounting is a show of dominance. It can also become excessive and lead to fighting. Fights can become vicious and even deadly.
If you are seeing this behavior, then these two should not be together. If they have not been spayed and you had hoped for them to bond, then they both need to be spayed first. Once spayed and healed, then introductions can be made (as if they've never met) and a bonding attempt can begin. They should not be together anymore until both spayed.