RustyB,
Polyurea is a group name just like lithium complex and calcium greases. There are many different versions that all fit the family descriptor. For polyureas there are two kinds which dominate and those are often referred to as tetra-ureas and di-ureas. Di-ureas are much more shear stable than tetra-ureas.
Compatibility will depend on type of PU. Compatibility for di-ureas are better than for tetra-ureas but if or not to mix two thickeners is not always a question of chemical compatibility. If you are converting from a product with excellent shear stability to a product with poor shear stability, leakage may falsely appear to be a result of incompatibility.
Good advise is to be cautious with thickeners having borderline compatibility such as PU. Run a compatibility test if you are uncertain.
You can also make a risk assessment of your application. If the product is to be used in an application with low shear and low strain incompatibility may not be an issue.
Be extra cautious in medium to high speed applications, vibrating housings, housings exposed to shock loads, high temperature applications and housings with high filling ratios >50% or housings without drain holes or double lip seals.
Stamina brand show fairly good compatibility with lithium complex greases.
Cheers,
GG