It seems like votes should be considered simultaneously to avoid complex alliances of the form: I will vote on B4 in the direction you like if you vote on B3 in the direction I like, but this is only possible in one direction WRT time. Having such an ordering and resulting negotiations means that some agents have an incentive to bargain for moving the location of a bill. It seems better to be able to make all such Bx vote for By vote trades. I'm not familiar enough with voting models to know the tradeoffs for a simultaneous system though.