I would often mix loads at the HS. However, this would only happen at crunch time at the semester end. I had a rule that the pots had to be completed by a certain due date, and then glazed by a certain due date. As such, sometimes the two due dates would cause a problem if the pot was not completely dry because of weather, size, or other reason. In these instances I did some crazy things. First as I would be firing glazes or bisques every day I would place pots that were not completely dry on the top of the kiln to rest while the kiln was firing-nothing heavy mind you.
Then these would get loaded in the next load, usually a mixed load. The mixed load would always be separated by shelves, usually a full shelf between with the bisque being in the bottom. The glazed wear would start on the level above the full shelf. I would fire the load like a bisque load and then go to glaze temp turning down the bottom elements after red orange heat. This would usually do quite well, and even though we the glazes would have a tough time on the higher temp bisque, it was doable when they were warm. I always told the kids the dangers of glazing over the vitrified clay, but if you know kids. . . In the end it worked, but was not the best of solutions for mid range stoneware. Don't do it at home, but then I never have that sort of crunch time.