Added brown gravy, canned veggies with the fluid, seasonings as well as beef bouillon to the crockpot when I put my first roast in.
I'm not a food safety expert, but if you cook it until the raw roast is completely cooked, the bacteria should be dead. However, the first roast and the vegetables may be overcooked at that point, so in the future I would recommend adding both roasts at the same time.
Yeah, I was going to take the first one out before I did the last. I'll probably keep the gravy part for my mashed potatoes I'm making but be rid of the veggies. If nothing else they did provide extra flavor! 😉