I think, though, in thinking this through with everyone, a consensus is developing wherein the ability to acquire en Epic Boon Feat at 19+ total character level is the intent, so -- based upon the conversation in this thread -- I am not with you that there is an exploit occurring here.
It's funny. I have enjoyed the multiclassing rules for 5th edition D&D better than any of the previous versions. You have expressed how negatively you regard them a few times here, so I know you disagree. But, to me, back in 2014, the writers of the game hit upon a way to provide for a way for players to fulfill all kinds of different character concepts through multiclassing, but without breaking the game. Further, it is simple. Different strokes for different folks I suppose. It seems with all of the Youtube videos on character building that use multiclassing, it is popular with others as well.
The reason I do not have any issue with there being certain ways of multiclassing that result in 2 Epic Boons at 20th level is that, first, we are discussing a game here and there are always funky little elements like that in a game (they are even fun to play with), and second, for those using multiclassing and feats for the past ten years, leveling up in certain classes to certain levels has always been a consideration if one wanted regular feats (unless fighter, one would want to hit 4th, 8th, 12th, or 16th in a class in order to be able to acquire feats). That was never an issue for me before.
So, right now, the leveling options for acquiring a second Epic Boon at 20th level are 12/8, 16/4, 12/4/4, 8/8/4, 8/4/4/4 (wild), and 4/4/4/4/4 (crazy). I am sure there are all kinds of interesting character class combinations that do not use those and I am sure not having a second Epic Boon is not that big of a deal for those players, especially given that Epic Boons continue to be available at 21st level and beyond.