Why would Zeneca followed by Pfizer/Moderna be ok but not the reverse? Because Pfizer/Moderna want to convince people to ditch Zeneca after their 1st shot that's why. By the same token Pfizer/Moderna are pushing for a 3rd jab, and vaccinating children, plus spreading fears one jab of J&J may not be enough.
When it comes to agressive salesmanship, mRNA vaccines are definitely the best.