Yonko and Admirals are top tiers both, but Yonko are clearly above, but not a difference like between a YC1 and an Admiral for example.
Yonko vs 2 Admirals is a debate because Big Mom could bring Kid and Law until this point. Meaning a Yonko vs 2 Admirals would end up with the 2 Admirals winning regardless but with serious injuries and barely being able to move (because like that will end up Kid and Law most likely).
Ofc this assuming they are Admiral lvl.
Anyway, even if they are not, they are close, and 1 Yonko vs 2 Admirals is still a "even" match, not a stomp, neither low or mid diff.