Try putting some treats in your pocket, bring the dogs together while you have treats in your hand, and treat both dogs when thry do not fight. Pet and praise both dogs at the same time. Keep doing this until they know that mom gives the treats and love when they don't fight. :) - Blu
Dogs fighting can be hard to resolve and Chihuahuas can be some of the worst. Is your female spay? If not, I suggest you get that done.
You need to watch them and try to determine what precipitates the fights. There will be signs from the aggressor like growling, baring teeth, hackles up, staring, etc. as soon as you see this, give a stern and loud correction or give a good poke to the aggressor or nudge with your foot to distract him. Keep some rolled up newspapers or magazines around and smack your hand or something with a loud NO or STOP!! Again, "before" a fight breaks out. You don't want to grab the front end of those dogs when fighting or you can get bitten.
Have either had obedience training? If not, I suggest you try that. Do you walk them? You should try walking them together regularly. Don't feed them too close to each other or unsupervised.
In the mean time, don't leave them alone together unsupervised if that means separating them in crates or with gates.
This may or may not be able to get resolved.
It might be a good idea to consult with a behaviorist. Perhaps your local Vet or a good dog training facility can suggest one.
Good luck.