adolescent dog and puppy

Hi!
You've got a lot on your hands with two young, rambunctious puppies!

Well, due to the size disparity alone, you're correct in your concern that Tiger could seriously harm Jack, even if he didn't mean to. Make sure that Tiger has lots of appropriate toys. Work with him (separately from Jack) on games such as "Find It" and teach him to "hunt" for food, toys or you so that he is directing a natural doggy instinct into a constructive and non-harmful behavior. You could also work on training a very strong "Drop It" cue, starting with a very low-value item (such as a boring toy or stick) and gradually working up to higher and higher value items until Tiger will reliably open his mouth and release whatever he's holding, on cue. Make sure both dogs are getting appropriate amounts of exercise. Walk and train them individually, not together - give them each private time with you.

Of course, Tiger is still just a puppy and enthusiasm could easily get the better of him until he develops some emotional maturity and self-restraint. Since this is the case, your most important job with both dogs is prevention and management. Look at the times that Tiger has grabbed Jack and determine what it was that triggered the grab. Were they playing and it got too exciting? You need to step in BEFORE that point and separate them for a little "cooling off" time (with a nice bully stick or Kong to chew and dissipate that excess energy). Did something else happen? Was Jack running away from Tiger? Was food or a high-value toy involved? Was Jack's yelp really one of pain, or just a communication to Tiger that Tiger had gone to far? How did Tiger react to Jack's yelp? It's definitely a scary thing in the moment that it's happening, so try to look at the occurrence as objectively and analytically as possible now.

Set the dogs and their environment up for safety and success. Keep Jack out of situations where he could get injured. If that means he can't play with Tiger right now, so be it. Keep Tiger out of situations where he can't succeed at good behavior. If that means he can't safely play with Jack, keep them apart. If you decide they can play sometimes, keep their interactions short and positive, supervise constantly, and separate them again before anything has a chance to escalate. If they don't tend to play by grabbing at each other's necks, have them wear their collars with either short leashes or long lines attached so you can safely separate them if necessary while keeping your hands out of bite range. Keep a citronella spray (SprayShield) or similar product on hand for absolute emergencies (such as Tiger is holding Jack and can't or won't let go at all).

As both dogs mature, and after Jack is neutered, and as they come to understand the rules and boundaries you are teaching them, they will become more reliable and it could become safer to allow them to be together longer, or all the time. I certainly know mastiffs, Staffordshires, Rotties, and other large, powerful breeds who live peacefully side-by-side with chihuahuas, poodles, Pomeranians . . . you name it. It's very do-able, but it's up to you to manage them and keep them both safe. If you're really confused and feel that you need some help with management, consult a positive trainer (I always pimp it, so here you go: APDT.com is a great resource that can help you find a trainer near you). A trainer can help you assess the situation and set up a behavior management and counter-conditioning program so that your dogs can coexist peacefully and safely.

Hope this helps! Best wishes to you and the boys!
Megan