Player B gets to choose exactly which of the 6 damage is prevented.
The two creatures deal damage simultaneously. Rule 510 deals with the combat damage step, and days among other things (emphasis mine):
510.2. Second, all combat damage that’s been assigned is dealt simultaneously. This turn-based action
doesn’t use the stack. No player has the chance to cast spells or activate abilities between the time
combat damage is assigned and the time it’s dealt.
So how does the prevention happen? Rule 615 speaks about prevention effects, and we find that the affected player (or the controller of the affected permanent) decides freely what damage to prevent if multiple sources deal simultaneous damage:
615.7. Some prevention effects generated by the resolution of a spell or ability refer to a specific
amount of damage—for example, “Prevent the next 3 damage that would be dealt to any target this
turn.” These work like shields. Each 1 damage that would be dealt to the “shielded” permanent or
player is prevented. Preventing 1 damage reduces the remaining shield by 1. If damage would be
dealt to the shielded permanent or player by two or more applicable sources at the same time, the
player or the controller of the permanent chooses which damage the shield prevents. Once the shield
has been reduced to 0, any remaining damage is dealt normally. Such effects count only the amount
of damage; the number of events or sources dealing it doesn’t matter.