The two tank approach may work, but you may have difficulty controlling that. Another option to consider is making a custom fluid using FluidProperties:* that has a specific heat profile similar to the phase change material. I'm envisioning a separate plant loop for the phase change tank with a heat exchanger to the regular loop, but again, you may have difficulty controlling all the parts. Another options is to use a normal tank with equivalent storage capacity and operate it within a narrow temperature range. Ultimately, you have to question what different the phase change material will make in the overall energy use, or whether it's just a way to pack more storage into a smaller volume.