Can't help with resolving this for you, but I do know that nearly all Jack Russells are convinced they are 15 foot tall and therefore can take anything on- they are truly mental!
My mum has one, and the bigger the dog, the more he goes for it!
Best try and exercise them away from other dogs, and don't let them get close to other dogs. A lot of dogs are twitchy when they are on leads, double that for your twon, and the fact that they are JR's seals it - keep them away from the dog population in your area.
Also have to agree. I have a JR cross and she used to be fine with other dogs. Well I say fine, she'll play for a bit but then it would get nasty as she was trying to be boss. Anyway, she had a fight with my dad's dog and ever since then she thinks she can take on the world. I now have to avoid other dogs due to as soon as they come running over to her or sniffing her she goes for them. I call it "little dog syndrome". You could look into a trainer but to be honest I just think its their nature. It is a pain as I would love to see my dog play with other dogs but it just isn't going to happen. I tend to take my dog to country fields where you wouldn't find too many dogs. My dog always goes for the bigger dogs as well.. i.e Rotti's etc.
Our Jack Russell, Che, is now a wonderful two year old. However from about 9 months till about 18 months he wanted to go for other dogs and the bigger the better. It was as if he wanted to put others in their place. He was the new man on the block. But at 2 he seems to be happy with himself and has settled down. We all love him to bits; he is such baby. Hopefully your two boys will grow out of this stage. I think socialising is better than avoiding other dogs though. Good luck
I agree with wee-janine, socialising is better. Have your pups been to puppy school or training class? If not, I would recommend it. Also, keep some treats in your pocket. When you see other dogs coming, say 'NO' or 'NO BARK' very sternly. After a few seconds when they haven't carried on then reward them with treats. Keep doing this and hopefully after awhile they will get the picture that if they shut up they will get a yummy