No. Those kind of bonuses and penalties do not stack. The rules for multiple players in a combat do address this issue:
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Originally Posted by The Rules
. If you are facing Grendel and a Dwarf helps you, the monster’s combat strength is reduced by 3 (unless you are also a Dwarf).
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The parenthetical statement implies, strongly, that once a penalty or bonus is applied, it does not get applied again if both players in the combat would get potentially get the bonus.