So here's another issue. The game will get abused in a new way - people without the book of mushrooms/stars will be able to get the mushrooms/constellations, because the other player have them, or the other way around - people with no finished quests will be able to get only 1 coef. 5 item, so the other player will be able to unlock it fast.
Basically they could talk using Skype, ICQ, Ventrilo or any of the hundreds applications that connect two people with headsets.