This has been in the game since at least A11, IIRC. In coop, if two people harvest a tree at the same time, a set of logs is created for each player harvesting, thus doubling the number of logs harvested. While not checked or verified, I suspect that the game is creating one set of logs for each player, such that if four players harvested a tree simultaneously, four sets of logs would be created. While this is nice for rapidly generating wood for bio-fuel, it probably should be fixed.
A little more detail: If a tree would normally drop one set of 3 logs when harvested by one person, when harvested by two people, two sets of three logs get dropped, one for each player. I suppose this could be listed as "How to double your log output in one easy step!"
