Sunk, I'm not sure "the other one" means "the wrong one". I'm not an epidemiologist, but if as Moore says there are no data on mix and match, then presumably nobody's stood up and said "You can't do that, it's dangerous."
No wonder really, as it's only a week or so since more than one vaccine has been available, so there's been little scope for trialling it. No doubt it will become more complicated as more come on line.
It may be as woofgang has suggested that any two would be usable together, like ibuprofen and paracetamol. Then again, maybe it would be like bleach and vinegar, a very bad idea.
But at the moment there don't seem to be any grounds to rule it out; the current position is just "We don't know".