Have you tried treats -showing that harness and leash are not that bad?
Start by letting them sniff the items -put them on the floor -let them "explore" it -sounds silly -but if they have any negative experience with it -you need to "break the pattern"
Speak with soft voice ,praise them dor sniffing both items then slowly put a harness on reward with praise and treats ,let them get used to a fact they are wearing it -give them few minutes before attaching leashes -reward with treats and praise
It will take time and patience as this isnt a easy fix -but it worth the trouble .
Ps you can use their own food as treats (then of course adjust their feeding quantities accordingly to the amount of dog food / treats you have used x
Dont get frustrated ,keep calm -they can sense it xx
And most importantly -well done for rescuing 2 unwanted babies xx rescues rock