Combining vaccines is disturbing. It might make sense in one example. I’ve learned that two doses of the AZ vaccine sometimes don’t work optimally because, after the first dose, the recipient had already become immune to the chimp adenovirus vector! A second dose of Sputnik...

... is rational as they’ve used a different viral vector. So no prior immunity.
Where it comes to combining, for example, Pfizer & AZ is immunologically unsound. Each is completely different so the second dose won’t “boost” the first. It’ll be as if someone was given two first...

... doses. That’ll yield inferior immunity compared with two doses of the same vaccine. 
Would someone intimate without the details of the proposed mixing trials let us know why this isn’t an issue?

It’s surely an easier thing to use available supply to match up people for the same second dose than to run a second very complex clinical trial.
Also these combination trials will probably focus only on raising antibodies. That won’t really even tell us about efficacy,

... as in the ongoing trials. 
They’ll be too small & too short to give us any useful information about clinical safety.
After 30y in pharmaceutical R&D, I’ve never seen anything quite as disturbing as what we’re doing with these novel technology vaccines.
Remember, those...

... under 70yo and in normal health are not at obviously elevated risk from this virus. Median age at death is well over 80.
Where’s the precautionary principle & medical ethics?