I think mixing vaccinations is the most ridiculous thing I've heard of. Imagine doing two different forms of stale reference manipulation in your genetic code and expecting the same result.
The fact there is more than one vaccination pushed out means that there is already uncertainty about the effectiveness of the process. That was the biggest indicator for me. They should be both be targeting the spike protein the same way otherwise that's just confirming to me the level of uncertainty in the knowledge about the virus's mechanisms.
This is why rushing a vaccine should not be a thing. It either works, or you are just guessing and pretending that you know it works.