If the oils in question are API licensed then they must pass ASTM D6922 which guarantees they will be miscible. This means they will not cause harm and the additives will not cause the mixture to gel or separate. This does not mean however that the resulting mixture will still carry any of the licenses, specifications or approvals that the original oils had.
Significance and Use
5.1 It is important that engine oils from different manufacturers be homogeneous and miscible with each other, because operators of automotive engines often do not have prior knowledge of the manufacturer of the oil that is currently used in their application, and engine failure can occur if oils are combined that do not stay homogeneous and function properly.
It gives you a mixture of two otherwise fully-formulated motor oils that may or may not still meet any of the requirements for the licenses, specifications or approvals the individual oils otherwise met. It also does not guarantee the mixture will still meet the winter rating of either of the individual oils.
It will not cause damage but it will not be an improvement either.