My personal recomendation is to NOT mix diferent oils. If you do not have another choice, in addition of what Pinkston posted, I sugest prepare 5 diferent mix samples and evaluate it in the lab. The 1st one 20% oil A and 80% oil B, the 2nd one 40-60, 3rd 50-50, 4th 60-40 and 5th 80-20. This will give you a better idea on what is going to happen in the system.
Once again, my recomendation is to NOT mix diferent oils.