I playtested with 4–6 players each session. They were very cautious players, and you can often flee from danger to regroup (and get your friend back in the game even sooner), so I don't think we ever saw multiple characters get killed in a single fight. If multiple characters did get killed, though, then yes, you could see them both boost the same roll. I don't really see a problem with that, personally.
The option to roll a d6 instead of CHA bonus was added because some players don't feel like they don't get to participate if they're just saying "I help them" and don't get to roll a die too. I encourage you to modify the rule at your table, though!