This is one reason why you need at least two circulators (I have three). The other reason is to be able to cook vegetables while you're also cooking meats. Veggies (especially the dense ones like potatoes, carrots, broccoli) generally have to be done at high temps, 180F-195F, for an hour or two to get to al dente or more well done. You can't expect to cook some steaks and then cook some veggies and be able to serve both at the same time with the intended results.
As for cooking two different products at the same time, if the temp is the same for both foods, why would you need to stop one cook before starting the other? If the difference in temperatures is only a few degrees, it should have little or no effect on a 30-hour cook to run the water bath a few degrees hotter for a few hours, because the focus of your long cook is breaking down tough muscle fibers rather than getting the meat to rare or medium rare or medium.