I don't think using a crop will help... if the dog wants to attack the other no amount of hitting you give to the dog will stop him from attacking. And ontop of that you will be in a bent position which will give the energy of joining the fight. Adding the hitting will just make it worse... I do agree however that the op needs to be alpha whatever female or male as that will help alot.
Now I am no expert at all but I have retrained fighting pits and other dogs and the one thing I learned is you never want to bend towards them when they are showing any signs of bad behavior. Just you bending into them shows you are encouraging the behavior. The easiest way I have found to calm them in a group of other dogs is to muzzle them (make sure they are sturdy muzzles that wont break) and let the dogs on leash. One dog starts walking the other dog walks behind so they have to smell the butt. Then switch until they both have done there smelling. Then let them loose and see how they do... I would still keep leashes on when loose so you can grab at them easily.
Edit: so you are not bending towards them you want to bend straight down ..does that make sense... instead of hunching over do more of a squat and keep your back straight..