Thats a good question. I know that some manufacturers like Torco offer a Synthetic Blend. Which I Torco's is a 60/40 blend of Synthetic and Mineral. I would imagine they use a some sort of special mixing procedure other than just dumping in some syn and min. in to a bucket and stirring, but maybe thats exactly what they do.
Mix away! Unless you have some castor oil in the mix everything is compatible and there are no issues.
Even though oils may be compatible you are not going to make a better oil by mixing. The mixture will at least be as good as the lowest rated product in the blend.