Greases are available with many different thickener types, base oils, and base oil viscosities. Mixing of these different types of greases can result in changes in characteristics, leading to a degradation of properties of either or both of the original greases. Extreme cases of grease incompatibility can lead to catastrophic failure of the components being lubricated.
That is why it is best practice to avoid mixing any types of grease. However, when it is inevitable, there are compatibility tests and guidelines available. Consult with your lubricant supplier and equipment manufacturer to ensure that the mixture will be acceptable.