I have not seen anything on the effects of such a practice, but here is my prediction. The cements have different heats of hydration and will cure at different rates. As such, any drying shrinkage that you would have from a single mix would be increased due to additional differential shrinkage. Aside from the cement, the components of the mix will also vary in quantity and/or type, so there are other interactions going on. My advice: don't do it (or if you do it, let us know what happens)