What is the basis for the production of heat-treated products with different materials and processes in the same furnace?

Different materials but the same technology, because the same technology can be done together.

In order to save cost and energy, it is generally called combined furnace. The method of rational utilization of the remaining space in the furnace.

What basis do you need? Workpiece A and Workpiece B are both carried out according to the original process, and Workpiece AB does not interfere with each other, so the combined furnace production is reasonable.