That's difficult to say without more information. Aggressiveness can be affected by many factors. The big ones are:
The gender of the dogs. Are they both male?
Neutering. Neutered dogs tend to be less aggressive, but neutering shouldn't be done before 6 months of age for health reasons.
Early socialization of the puppies. If they were introduced to lots of other dogs in the first 10-12 weeks of life, they are more likely to get along together.
Level of obedience training.
If both puppies are male, I'd say it's not likely they will get along, especially before they are neutered. Both JRTs and Chihuahuas can be quite territorial and sometimes not very tolerant of other dogs.
