Yevamot 24 - He does not know whom he married
If a man betrothed one of two sisters but does not know which of them he betrothed - he needs to give a divorce (Get) to each one. He cannot live with any one of them - because perhaps he betrothed the other one, and this one is prohibited to him as his wife's sister.
If he dies before divorcing, his brother performs a chalitzah with both - on the eventuality that each can be his brother's wife.
If he had two brothers, then one of them must do a chalitzah with one, but the other brother can marry the other sister. This is permitted according to both possibilities: if her sister, who got the chalitzah, was really the wife - then the bond is now dissolved, and he can marry the other sister. And if her sister was not the true wife, then the chalitzah meant nothing, and the brother is now marrying his deceased brother's wife, that is, doing a yibum.
However, they cannot both marry, each one to a sister. The one who marries the true wife is doing a yibum, and that is fine. But the other one - he is marrying a women to whom there is a yibum-bond, something we discussed earlier . Still, if they don't consult and just go ahead and marry, they can stay married - because each one can claim that he did the yibum, and it is the other brother who was in violation, and now anyway, the yibum bond is no longer there.
The eldest brother is the one who should do the yibum, but if the younger one has preceded him - he has acquired the mitzvah.
Art: The Cameron Sisters by Stephen Catterson Smith