Why Not Get Two Different COVID-19 Vaccines?

I've been wondering this for a long time. Why not, say, get one Pfizer shot and one Moderna shot? Or one J&J shot and one AstraZeneca shot? Or whatever? I mean, it's always sounded to me like a good idea to emphasize getting everybody over 30 or whatever one shot before starting in on the second round...but I know nothing about this so, I don't even deserve an opinion...but, anyway, mightn't it be a good idea to go with two different shots rather than two of the same? There are undoubtedly brilliant experts writing hundreds of pages on this somewhere...and running across ignorant shit like this on the interwebs probably makes them want to smash their monitors...