The formulations are developed by engine testing with the complete oil charge of one oil. Even Mobil 1 doesn't overformulate; there's enough of a certain additive to do the job, and that's about it. Many times the additive concentrations are not linear with performance; that is, some is OK, a little more is a lot better. Therefore, having two formulations that deliver half the additive may not be as good as one formulation with the intended amount.
The only mixing that is "tested" is a bench test to make sure that a combination does not cause haze, additive dropping out, or some other chemical incompatibility. It has nothing to do with engine performance.
Lastly, even within the same brand, the marketer may use different additive packages. The additives in the synthetic oil are probably not the same as in the mineral; even the additive in the new SAE 5W-20 grade may not be the same as in the SAE 5W-30.
Save the extra in the trunk for emergencies, but for the few bucks the oil cost, it's not worth the risk of finding out whether the two brands are happy together.
