I have small amounts of two different lubricating greases. Can they be mixed in my application?

It is best practice to never mix different lubricating greases.  Greases should not be mixed because the thickeners, the lubricating fluids, and the additives in different greases may be incompatible.  Some grease suppliers publish compatibility charts; those charts are at best a rough guide.  If greases must be mixed, as during a product change-over in industrial equipment, compatibility testing, as described in ASTM D6185, should be carried out to determine to what degree the products may be compatible.