פירוש על קידושין 100:12

Rami b. Hama uses the verse that prohibits marrying two sisters to teach that if he marries them (or tries to) simultaneously, neither of them is considered married.

Rava points out that the verse prescribes karet (cutting off) for anyone who marries two sisters. But if the verse is talking about someone who tries to marry two sisters simultaneously and the betrothal is not valid, then why should he (or they) receive any punishment? He (and they) did not do anything.
Therefore, he explains that the mishnah follows Rabbah who said that when an act, such as marrying two sisters, cannot be performed consecutively, it can also not be performed simultaneously. If he tries to marry two sisters at the same time, neither are married.