Bounty Board’s last ability looks for any bounty counter on any creature, not just one from that Bounty Board. For example, say players A and B each control a Bounty Board and player C controls a creature with a bounty counter on it (no matter how that bounty counter got there). When that creature dies, the ability each Bounty Board will trigger. Each of those abilities will cause players A and B to draw a card and gain 2 life, for a total of two cards and 4 life for each of them. Player C is left only with a creature to mourn and revenge on their mind.