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Hi. Welcome to Just Answer. My name is***** I'm glad that you know that onions are not good for dogs. How much does each of these girls weigh? Was this just a single regular sized onion ring?
How long ago did this happen?
Ok. The generally accepted number for onion toxicity is 5 gms of onion/kg of body weight to be toxic. How many grams were eaten and by which dog? That is the unknown. The safest thing to do would be to induce vomiting in both and see what comes up. It could detoxify the one that did get into them. The greasy onion rings of that volume could also cause pancreatitis in those that are more sensitive. Here is a link on inducing emesis.
The 35 pound dog would have to eat 79 g of onion to get to that range. Onion toxicity causes lysis of red blood cells. It would typically show in a couple days as being weak and most likely having darker / to blood colored urine.
Getting some dogs to vomit is very difficult to impossible. I'm glad it was the bigger one that decided to partake in them. The diarrhea now isn't that surprising as that rich "snack" can really throw off the intestinal tract. At home, you could try a bland diet of 1/3 boiled chicken and 2/3 boiled rice - 1 cup per 30 pounds three times a day as this can sometimes be enough to settle things. The quicker step would be to let the vet's office know and they might prescribe an antibiotic called metronidazole to help out.