We also have two rescue dogs. Our older Labrador ignored the new puppy we brought into the house for a long time, as if she wasn’t there. Then one day, he suddenly started trying to play with her.
Since then (they are now 10 and 3 years old), he’s been very tolerant of her, even allowing her to push him around. Sometimes he shows her that enough’s enough, by growling and showing his teeth, and she backs off. I would recommend that you have the newcomer neutered or spayed ASAP, if that’s not already been done.
If the dogs are already together at your home, the best thing is to leave them to work things out themselves. They will establish their own pecking order. If anyone’s considering bringing a second rescue dog into their home, it’s usually best to introduce them on neutral territory. We’ve had to turn down two rescue dogs in the past - on one occasion, our existing dog took a dislike to the rescue one as soon as she met it, and the second time it was the other way around.
As you well know, many rescue dogs have been ill-treated. If you don’t get them straight from their mums at 8 weeks of age, there’s generally some challenge or another to deal with. Our Lab has become very snappy around other people when he’s on the lead, yet he’s completely submissive in the home - he’s super-protective of me. What happened to him in the 4 years before we adopted him, we’ll never know. Good luck with your dogs!