Since there is still alot of hissing and growling, I wouldn't leave them alone unsupervised. If you're only going away for a couple of days, just leave plenty of food and water (water especially since it's summer and hot out) out for them in their respect places. Cats are natural grazers. They won't gobble up a big bowl of food at one time. Remember, cats don't like change. And since they are still in the process of getting used to each other, I wouldn't take the chance. Just my opinion from years of experience.