Somewhat off topic but in theory, would it be possible to have jab #1 be Moderna's and jab #2 be Pfizer? Would it be ethical to test this? Is there anything inherent in either's approach that would make this dangerous?
That would probably not work the same way.
The point of a booster shot is to make the immune system "think" it's been infected by the same intruder twice, so it steps up permanent defences to a more serious level.
Both the Moderna and Pfizer vaccine produces replicas of the virus "spike protein", but with (presumably) different mRNA formulas.
So if their different versions are similar enough that the immune system sees them as "this shit again!", which would work as a booster shot, or as an unrelated infection, is impossible to say. At least for me, who is (again) unqualified to talk about this stuff.
That said, two vaccines would at minimum be better than one.