Jack bites Tomy. Help!

Hello everyone.
My name is Lazar, this is my first post on this forum, and I created an account because I really don't know what to do.
My brother bought a staffy, Jack, when he was 3 months old, and he was a biter.
Now he doesn't bite me or him, or anyone else except my little spaniel named Tomy.
The problem is this: I bought a spaniel, Tomy, which is almost 2 months now, and Jack is 5 months old.
Tomy lives in a house and Jack is living in the yard. When I introduced Tomy to Jack, Tomy was scared but later he started waving his tail, while Jack was waving his tail all the time. I gave them some time, about a week before I removed the barrier between them and both of them were on a leash (I know that Tomy is too small for the leash, but he was just 20 minutes on it and I did it because I didn't want anyone to be jelaouse). The problem is that today when I thought that Jack and Tomy can be close one to another, Jack tried to bite Tomy.
Tomy is really small dog and I'm affraid that only one bite could be fatal to him.
I really need some advice on how to train Jack not to bite any dog.
Note that Jack socialised with neighbour dogs and didn't try to bite any of them. All of those dogs are at least 1 year older than him.

Why is one in the yard and the other in the house ??

am not much help I am afraid, I have never had two dogs together.
Why don't you come and introduce yourself here.
https://staffy-bull-terrier.niceboard.com/f7-new-member-introductions  

Perhaps jacks wondering why he's outside and tomy is allowed in the house?

Might be completely wrong but I suspect jealousy as my first cause , my boy is not so happy that princess gets on my pc chair whilst he stays on the floor ( beds and sofas are a free for all btw ) so the outside / inside thing is a bit more major

Dave wrote:Might be completely wrong but I suspect jealousy as my first cause , my boy is not so happy that princess gets on my pc chair whilst he stays on the floor ( beds and sofas are a free for all btw ) so the outside / inside thing is a bit more major

I thought this too  

why is he kept in the yard?

Could be jealousy, Daisy is very fussy just like my sisters dog Grace, and Grace gets very jelous of this, and tries to keep her away from us sometimes.