Which dog was there first ~ Sadie or Bentley?
She might be 'resource guarding' and YOU are the resource she is guarding.
I would definitely keep them separated when you are not home and maybe when you are at home - until things calm down. I second the suggestion of a trainer. I like your idea of more walks to tire her out a bit. Can you walk them together to help build their bond?
IF no blood is being drawn - the fight is probably more noise and for show than anything else. But, still, it's upsetting and no one wants to live like that all the time.
I'd also start implementing NILF for Sadie ~ well both, I guess. Start teaching her some basic manners.
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